Earlier this week, we projected the five best offenses for the 2015 season. But assessing defenses is more difficult. Whereas a good quarterback often covers up defects, there are few defensive players at any position that have a similar effect on their teams. The rare exceptions, like the Houston Texans' once-in-a-generation defensive end J.J. Watt, have an impact by working from a multitude of positions and places on the field.

Here are the top five units for 2015, as things look right now.

5. St. Louis Rams

The Rams have employed some competitive defenses during Jeff Fisher's tenure as head coach, but none that could be accurately described as elite. While the Rams possess one of the more loaded defensive depth charts (particularly in the trenches), that talent hasn't always manifested as strong play. For one reason or another -- an ill-timed injury to Chris Long, inconsistent cornerback play -- the defense has yet to pull together and reach its potential. Last season, the Rams ranked 17th in points per game (22.1) and 16th in takeaways (25).

That should change this year. St. Louis' front four, arguably the best in the league entering the offseason, added defensive tackle Nick Fairly to the mix. Fairley might have been the Detroit Lions' best lineman after Ndamukong Suh in 2014, but he expects to serve in a reserve role in St. Louis, rotating in to keep Aaron Donald and Michael Brockers fresh. Not only are the Rams better protected against injury, but the improvement in the pass rush should make life easier for the secondary.

Furthermore, a look at the advanced metrics suggests that the defense might have performed better than perceived last year. On a per drive basis, the Rams ranked an impressive sixth in points allowed (1.63). With more firepower on defense (and Todd Gurley helped the offense eat up clock), the total number of points should fall in 2015.

4. Baltimore Ravens

Considering all the hurdles that faced the Ravens last year -- an extensive list before even considering the black clouds brought on by Ray Rice -- it's quite astounding that they performed as well as they did. This holds particularly true for the defense, which lost two of its top three cornerbacks for roughly half the season and also played without top cover man Lardarius Webb for a spell. Still, the unit finished sixth in points allowed per game (18.9) and 10th on a per drive basis (1.71).

The undermanned secondary returns to full strength this year, augmented by free agent additions Kyle Arrington and Kendrick Lewis. In the front seven, versatile pass rusher Pernell McPhee and longtime stalwart Haloti Ngata have departed, but the Ravens might actually improve in those areas. 2014 second-round pick Timmy Jernigan steps into Ngata's role along the defensive line after a promising rookie campaign. He'll play beside Brandon Williams, a grizzly bear of a football player that has quietly established himself as one of the league's premier nose tackles. While McPhee's ability to rush from a multitude of positions won't be easily replaced, Baltimore has the horses to adjust. Terrell Suggs remains a field tilter even at his age, while Courtney Upshaw and Elvis Dumervil provide quality balance along the edge. Rookie Za'Darius Smith should also see some playing time, especially later in the year.

3. Houston Texans

A year after J.J. Watt nearly become the first defensive player to win league MVP since Lawrence Taylor in 1986, it sure feels like he can single-handedly drag any defense into the upper reaches of the NFL. Not that the Texans failed to put playmakers around him, but the way Watt savages offensive lines and terrorizes quarterbacks and offensive coordinators in equal measures, it's easy to understand why he monopolizes the conversation.

Houston's defense did quite well for itself last year, finishing seventh in average points allowed (19.2) and fourth on a per drive basis (1.53). However, unlike the other units mentioned so far, the Texans excelled in creating takeaways, leading the league with 34 combined turnovers. While some of that production is unpredictable -- fumble recoveries vary wildly year to year -- interception totals have proven somewhat more repeatable. In that category, Houston finished third in the league with 20.

To improve the defense, the front office added veteran lineman Vince Wilfork to eat blocks in the middle as well as mentor young nose tackle Louis Nix III. At linebacker, the team re-signed edge rusher Whitney Mercilus and drafted Mississippi State's Benardrick McKinney to work inside. The secondary should also get a boost from the arrival of safety Rahim Moore and the departure of the mistake-prone D.J. Swearinger.

The wild card in all of this is former No. 1 overall pick Jadeveon Clowney. If he recovers quickly from offseason microfracture surgery, the Texans could feature one of the scariest pass rushes in the league. If not, they should still perform well, but Clowney's rare size and athleticism could elevate the defense to another level.

2. Buffalo Bills

In 2014, arguably no defensive front four did more damage than that of the Bills. The group, a veritable Pro Bowl lineup of Mario Williams, Marcell Dareus, Kyle Williams and Jerry Hughes, pushed the team to a No. 1 finish in sacks with 54. According to Pro Football Focus, Buffalo also finished among the league's best in total pressures with 287. Unsurprisingly, the pass rush had a direct impact on the scoreboard, as the team gave up the fourth fewest points per game (18.1) and the fewest per drive (1.41).

The star-studded defensive line returns for 2015, though the NFL suspended Dareus for the season-opener against the Indianapolis Colts. The back seven features almost exclusively young players, but linebackers Preston Brown and Nigel Bradham established themselves as quality starters a season ago and should continue to improve in 2015. The secondary looks a little more average, but Duke Williams is an above average free safety and second-round pick Ronald Darby has star potential.

Buffalo's defense should also get an added boost with the arrival of head coach Rex Ryan. In his previous stint with the New York Jets, Ryan regularly turned out terrific units that ranked among the league's elite. Already, the coach's trademark bravado appears to have caught on with his charges, as Dareus and several other Bills believe they can become the greatest defense ever. That appears unlikely, but the title of best in the league doesn't seem impossible.

1. Seattle Seahawks

Is there really any debate? The Seahawks finished each of the last three seasons with the top scoring defense, and it wasn't particularly close. In 2014, Seattle gave up just 15.9 points a game, almost two points better than the next best unit.

And the numbers hardly tell the full story. Opponents regularly avoided Richard Sherman. Eventual MVP Aaron Rodgers did not attempt a single pass against him during the season opener on the Packers' way to a 36-16 drubbing. Those that did test Sherman were usually made to regret it, as the All-Pro held quarterbacks to 48-percent completion and a passer rating of 39.8, according to Pro Football Focus. The results were much better passing against Jeremy Lane (67.4 passer rating) or Byron Maxwell (78.5).

Even through injuries, the Seahawks made life hell on offensive linemen. Michael Bennett clowned blockers from all along the defensive line while Cliff Avril enjoyed similar success at defensive end. At the next level, underrated linebacker K.J. Wright had his best year to date and Bobby Wagner garnered MVP consideration.

Seattle lost Maxwell to a six-year, $63 million offer from the Philadelphia Eagles, but otherwise the defense returns the same key figures from a season ago. Bennett's holdout threats do warrant some concern, but as long as he reenters the fold the Seahawks have little to worry about.