We live in an age of football obsessed with big numbers on offense, but with the increased scoring comes an increased emphasis on solidifying defenses. Plenty of defensive coordinators are commanding high salaries now as championship contenders try to solve the spread/tempo revolution of the last decade-plus. Eight of the last 10 national champions have finished in the top 10 nationally in defensive yards per play, and all but one has placed in the top five of Football Outsiders' S&P+ defensive ratings.

Most vacant head coaching positions are being filled with offensive minds, but you can't win a championship without a complete team. So, with the 2015 season just a few weeks away, let's take a look at what could be the top 25 defenses this fall.

1. Alabama. There's no doubt that the Crimson Tide have sported some cracks the last couple seasons. In last year's Iron Bowl, they beat Auburn 55-44, allowing 456 passing yards to Nick Marshall. In the Sugar Bowl, they lost 42-35 to Ohio State, allowing 230 rushing yards to Ezekiel Elliott, in addition to struggling to handle Cardale Jones. Nick Saban is an expert secondary coach, but the secondary has been a weakness, particularly against teams that push the tempo and spread the field. Still, Alabama's faults need to be put into context. It's all relative.

By almost any measure, this was still a good defense last year. Alabama ranked sixth in points allowed, third in Football Outsiders' defensive S&P+ ratings, 18th in yards per play allowed, fourth in run defense and 30th in defensive passer rating. Landon Collins, Trey DePriest, Nick Perry and Xzavier Dickson are gone, but a loaded defense return, particularly up front, thanks to top recruiting class after top recruiting class. Nobody can come close to matching the Tide's depth along the line where A'Shawn Robinson, Jarran Reed and Jonathan Allen start on the line, with a deep rotation of talent behind them. Reggie Ragland is a star at linebacker, and Reuben Foster may join him. Losing Collins is a big blow to the secondary, but cornerback play should be improved with the combination of senior Cyrus Jones and sophomore Tony Brown, who experienced expected lumps as a true freshman but has a high ceiling.

Alabama could use a more explosive pass rush, and there's no guarantee that the secondary will shake its recent issues, but the middle of the defensive front alone should be enough for this unit to be dominant. Nobody can match what Alabama has in the trio of Robinson, Reed and Allen, along with the ridiculous depth behind them.

2. Virginia Tech. The Hokies drew Ohio State at the perfect time last year: It was the second game of the season for a Buckeyes team with four new offensive line starters and a freshman quarterback unexpectedly in the lineup. The Hokies finished with 11 tackles for loss and seven sacks in their 35-21 road win, and Ezekiel Elliott ran for a season-low 32 yards. They ended up playing solid defense most of the year, finishing 14th in points allowed, 14th in defensive passer rating, fourth in sacks and 39th against the run. While leading tackler Kyshoen Jarrett is gone at safety, this will be a better defense in 2015. It's deeper and more experienced, with one of the nation's best pass rushes behind ends Dadi Nicolas and Ken Ekanem; three All-ACC caliber tackles in Luther Maddy (returning from an injury), Corey Marshall and Nigel Williams; a rising star at inside linebacker in Deon Clarke; and a loaded secondary led by All-America cornerback Kendall Fuller and Brandon Facyson, who ended up redshirting last year after struggling with injuries.

With such a potent pass rush and lockdown cornerbacks, nobody is going to throw on the Hokies, and they're also stout up the middle against the run. Longtime defensive coordinator Bud Foster is a master at bringing different blitzes, and there are very few offenses on the Hokies' schedule that will be able to handle it, assuming Virginia Tech stays healthy this season. With a little bit of improvement on offense, this defense can bring Virginia Tech back to its former familiar position as ACC Coastal favorite.

3. Ole Miss. No defense is more fun to watch than Ole Miss'. Whereas Alabama acts as a physical brick wall up the middle, Ole Miss attacks from all angles under the Landshark moniker. The Rebels are smart, explosive and athletic, and they fly to the ball. They finished first in defensive SRS last year, first in points allowed, seventh in yards per play and eighth in tackles for loss per game. There are some big holes to fill in the back seven -- most notably cornerback Senquez Golson, safety Cody Prewitt and linebacker Serderius Bryant -- but this unit still boasts a ton of talent, thanks to Hugh Freeze's recruiting successes.

It all starts with former No. 1 recruit Robert Nkemdiche, a freakish 280-pound athlete at defensive tackle. He hasn't posted big numbers and is still developing consistency, but he's a monster to handle in the middle, opening things up for the playmakers around him. Nkemdiche can line up anywhere up front, and this entire defense is adaptable. Safeties Tony Conner and Trae Elston also move around the formation, capable of playing hybrid safety/linebacker/nickel roles. Defensive end Marquis Haynes had 7 ½ sacks as a freshman. And defensive end C.J. Johnson is also showing off his versatility by making the rare transition to middle linebacker. The defense can carry this team a long way if the situation in the offensive backfield finds solutions.

4. Ohio State. The individual talent here is staggering, and the Buckeyes defense -- like the offense -- got much better over the course of the 2014 season. Losing defensive tackle Michael Bennett and cornerback Doran Grant is tough, but this unit is loaded with stars. While Ohio State finished 26th in scoring defense and 25th in yards per play, it ranked second in Football Outsiders' defensive S&P+ ratings. He'll miss the first game because of a suspension, but defensive end Joey Bosa is the best player in the country and the possible No. 1 pick in the draft. Linebacker Darron Lee and safety Von Bell became breakout stars in the second half of the season. Linebacker Joshua Perry and tackle Adolphus Washington are proven seniors. Linebacker Raekwon McMillan and cornerback Eli Apple are rising stars. Seven starters return to this unit, which also has a stellar coaching staff with co-coordinators Chris Ash and Luke Fickell, plus ace defensive line coach Larry Johnson. The individual talent is unfair, and this became a more cohesive unit late in the season, as we saw in the playoff.

5. Penn State. The Nittany Lions had one of the biggest offseason signings in the sport in giving coordinator Bob Shoop a big raise after LSU expressed interest in him. After years of stability, Shoop was Penn State's fourth defensive coordinator in four years. His defense was the backbone of James Franklin's successful Vanderbilt teams, and it kept the team afloat last year with the offense struggling mightily with scholarship restrictions. Penn State finished seventh in points allowed, third in run defense, second in opponents' passer rating, third in yards per play and 20th in tackles for loss per game. There are a few key players to replace -- particularly star linebacker Mike Hull and defensive back Adrian Amos -- but this unit remains loaded, and it finally has coaching continuity again. Anthony Zettel and Austin Johnson form perhaps the nation's best tackle combination, setting the tone for a defense that is really strong up the middle, with Nyeem Wartman-White sliding into the middle linebacker role and the underrated Jordan Lucas shifting from cornerback to safety, where he's joined by soon-to-be-star sophomore Marcus Allen.

6. LSU. After losing so many key players early to the NFL draft over two years, LSU was bound to take a step back last year. The defense got better over the course of the season, though, finishing 12th in Football Outsiders' S&P+, 18th in yards per play and third in defensive passer rating. There are some problems here, with defensive coordinator John Chavis bolting for Texas A&M and a lack of defensive line depth compared to past years. New coordinator Kevin Steele does, however, inherit a star-studded secondary led by cornerback Tre'Davious White and safeties Jamal Adams and Jalen Mills. Linebacker Kendell Beckwith became a star when placed into the starting lineup. And there is a high ceiling for the defensive tackle duo of Christian LaCouture and Davon Godchaux. Pass rush is a concern with questions at defensive end after a mediocre output last year, and Steele has huge shoes to fill in replacing Chavis, but LSU continues to recruit well. With a secondary full of playmakers, LSU has the talent to compete with anyone in the SEC again.

7. Florida. For all of Will Muschamp's faults as a head coach, he left new coach Jim McElwain plenty to work with on defense. The dire situation on offense may take a few years to work itself out, but Florida can at least give itself a chance to win thanks to its sturdy defense. Despite their 7-5 record, the Gators ranked fifth in defensive yards per play last year. Losing end Dante Fowler is tough, but otherwise this group is loaded: Vernon Hargreaves is the best pure cover corner in the nation, end Jonathan Bullard is poised for an All-SEC season and players like sophomore cornerback Jalen Tabor and true freshman defensive end Cece Jefferson could quickly become stars. The big issue is the health of middle linebacker and leading tackler Antonio Morrison, who has undergone two knee surgeries since suffering an injury in the Birmingham Bowl, with his status in question for the season. That's a crucial injury, but with a solid line and one of the nation's best defensive backfields, this will continue to be a top defense, especially since McElwain made impressive hires in former Mississippi State defensive coordinator Geoff Collins and former Miami head coach Randy Shannon to run the unit.

8. UCLA. This unit should be better than it has been, and it may become more consistently great under new coordinator Tom Bradley, who churned out high quality defenses on a yearly basis at Penn State. Bradley certainly has plenty of building blocks to work with. Myles Jack is a superstar at linebacker, although finding a replacement for tackling machine Eric Kendricks won't be easy. Defensive tackles Kenny Clark and Eddie Vanderdoes are rock-solid up the middle, Deon Hollins had nine sacks last year and the secondary is deep. Like the offense, the Bruins defense can have some frustrating lapses, but the depth chart is stocked with proven upperclassmen, giving this a chance to be the Pac-12's best defense, and giving UCLA the chance to compete for a playoff spot.

9. Louisville. Bobby Petrino may be one of the sharpest offensive minds in the country, but defense is carrying the Cardinals right now, with Todd Grantham building an excellent unit upon his arrival. Louisville ranked 11th in yards per play, 10th against the run and fifth in defensive passer rating last year. Now, there is a chance there is a drop-off. Only three of the top 10 tacklers return, with key players like pass rusher Lorenzo Mauldin, interception ace Gerod Holliman and leading tackler James Sample gone. But there is a core of proven players up front, and reinforcements are available elsewhere. Defensive lineman Sheldon Rankins is an underappreciated star, while the linebacker combination of Keith Kelsey and James Burgess is terrific. Now, we'll see what happens with an influx of transfers: pass rusher Devonte Fields, who was the Big 12's defensive player of the year as a freshman at TCU in 2012, arrives from the juco ranks (although Petrino is currently working on getting Fields into proper shape, apparently). And the secondary is bolstered by two Georgia transfers who previously played for Grantham: cornerback Shaq Wiggins and safety Josh Harvey-Clemons. It remains to be seen if all the pieces will gel, but the individual talent is primed for big things. The pieces will, however, need to fit together in a hurry, as Louisville opens with a brutal trio of games against Auburn, Houston and Clemson.

10. Boise State. The Broncos are nearly a unanimous pick to win the Group of Five's New Year's bowl bid again, despite the fact that they're replacing their starting quarterback and workhorse running back. Why? Nearly everyone else is back, including the makings of a very good defense. There were some lapses last year, most notably a stunning inability to stop New Mexico's ground attack in a 60-49 win. But the Broncos were also lights-out late in the year (while Arizona put up 492 yards in the Fiesta Bowl, it averaged only 4.64 yards per play). Boise State's depth can't match the best Power Five defenses, but its starting 11 has just about everything you could want. Kamalei Correa is an impact pass rusher who had 12 sacks and 19 tackles for loss. Linebacker Tanner Vallejo had 99 tackles and 15 ½ tackles for loss. Safety Darian Thompson earned All-America honors, picking off seven passes, and cornerback Donte Deayon was right behind him with six. The Broncos gave up too many big plays, but a more experienced unit will likely give up fewer in 2015, and few teams are as proven in terms of making big plays, whether it's forcing turnovers or generating negative plays.

11. USC. Replacing Leonard Williams is impossible, but the Trojans are deeper on defense, another year removed from scholarship restrictions, and they have impact players in cornerback Adoree Jackson, safety/linebacker Su'a Cravens and nose tackle Antwaun Woods.

12. Michigan State. It's hard to worry about the Spartans defense, even after losing ace coordinator Pat Narduzzi. Mark Dantonio is still here, with a top-notch defensive line (Shilique Calhoun, Lawrence Thomas, Joel Heath, Malik McDowell) and an underrated playmaker at linebacker in Ed Davis (**UPDATE: On Aug. 12, Michigan State announced that Davis suffered a season-ending knee injury in practice). It feels strange to say after recent production, but the question here is the secondary, which loses Kurtis Drummond and Trae Waynes and has unproven cornerbacks.

13. Georgia. The Bulldogs made clear strides in Jeremy Pruitt's first year as coordinator, and they'll take a another step forward this year. The linebacker trio of Jordan Jenkins, Leonard Floyd and Lorenzo Carter could be unblockable, and the secondary is on the rise.

14. Wisconsin. New coach Paul Chryst got coordinator Dave Aranda to stay, which is big news for a team that has dealt with surprising coaching instability. There's turnover up front for the second year in a row, but this quickly turned into a great unit last year. Safety Michael Caputo and outside linebacker Vince Biegel are two of the Big Ten's best defenders.

15. Texas. The offense remains a mess, but the Longhorns will find themselves in the top 10 defensively most years. Charlie Strong is a top defensive mind, and they'll be strong again this year, although there will be growing pains with six of the top seven tacklers gone, including Malcolm Brown, Cedric Reed, Jordan Hicks and Quandre Diggs.

16. Baylor. Will this be a shutdown unit? No, the defense is always going to be well behind the offense. Still, the Bears have become a playoff contender partly because of the strides the defense has made over the last few years under Phil Bennett. The defensive line is one of the nation's best, with Shawn Oakman, Andrew Billings, Beau Blackshear, Jamal Palmer and K.J. Smith, while linebacker Taylor Young, cornerback Xavien Howard and safety Orion Stewart can all be All-Big 12 players.

17. Stanford. The Cardinal didn't get enough credit for their phenomenal defense last year when they fell to 8-5. Strangely, this should be a much better team in 2015 despite inevitably taking a step back on defense. Only four starters return, with stars Henry Anderson, Jordan Richards, Alex Carter and A.J. Tarpley gone. This will still be a strong defense behind Blake Martinez and Peter Kalambayi, but there is definitely rebuilding to do.

18. Tennessee. The Vols ranked only 46th in defensive yards per play, but there's a lot to like here as more depth is developed. Derek Barnett was one of the nation's top pass rushers as a freshman, and players like cornerback Cameron Sutton, linebacker Jalen Reeves-Maybin and pass rusher Curt Maggitt make this a unit capable of big things.

19. Utah. The Utes have developed a physical identity, and there's plenty to like on defense, even without All-American end Nate Orchard. Hunter Dimick had 10 sacks opposite Orchard last year, Jared Norris is terrific at middle linebacker and sophomore nose tackle Lowell Lotulelei is following in the footsteps of his brother Star, a 2013 first-round pick. Kyle Whittingham lured John Pease out of retirement to run the defense after coordinator Kalani Sitake bolted for Oregon State.

20. Oklahoma. The Sooners underperformed in many areas last year, including with a maddeningly inconsistent defense. It wasn't because of a lack of talent. Coordinator Mike Stoops has a top linebacking corps at his disposal, led by Eric Striker, and cornerback Zack Sanchez had six interceptions last year and has game-changing ability.

21. Arizona State. This unit experienced a ton of turnover last year and was predictability unstable, giving up far too many big plays and posting some ugly performances. With that said, the Sun Devils lost only three games, and now they return nine starters on defense. Replacing safety Damarious Randall and end Marcus Hardison is tough, but Todd Graham has the playmakers to make his aggressive approach work, led by tackle Tashon Smallwood, cornerback Lloyd Carrington and safety Jordan Simone.

22. Auburn. The Tigers may have ranked 11th in the SEC in defensive yards per play, but there are plenty of reasons to be confident in a turnaround. New coordinator Will Muschamp consistently produces great defenses, and the cupboard is far from bare. Linebackers Kris Frost and Cassanova McKinzy decided to return, tackle Montravius Adams and cornerback Jonathan Jones can be stars and defensive end Carl Lawson returns from a torn ACL to boost the pass rush. Add in Georgia transfer Tray Matthews at safety and five-star freshman end Byron Cowart, and there's a lot to like.

23. TCU. We can't expect the Horned Frogs to instantly reload on defense without any issues, although Gary Patterson does generally deserve the benefit of the doubt. TCU led the Big 12 in defensive yards per play (4.66), and while it returns only five starters and lose six of their top seven tacklers, the unit is unlikely to fall too far. Patterson is a top defensive mind, and there are enough pieces left on the defensive line (Davion Pierson, Terrell Latham, James McFarland) and in the secondary (Derrick Kindred, Ranthony Texada) to have confidence that the transition will go relatively smoothly.

24. Notre Dame. This a leap of faith, given how badly the Fighting Irish fell apart last year, but then again they're going to be deeper and can't possibly run into the same amount of bad luck. Injuries and suspensions took their toll last year, and now, in the second season under Brian VanGorder, there is a lot to like. The NFL loves linebacker Jaylon Smith, and playmakers like tackle Sheldon Day and cornerback KeiVarae Russell (coming off a suspension) can be stars as well.

25. Temple. Yes, really. Matt Rhule has built an AAC contender thanks to a great defense that returns almost everybody on the depth chart. Senior Tyler Matakevich is a machine at linebacker, and there's proven experience all around him. The Owls gave up only 17.5 points per game last year and held potent Cincinnati and East Carolina offenses to a total of 24 points in November.

* * *
Contact Matt at matt.brown5082@gmail.com and follow him on Twitter @MattBrownCFB and Facebook.