By Cliff Corcoran

The Astros' rotation projects to be the best in baseball in 2018 thanks to the weekend addition of former Pirates ace Gerrit Cole. Cole joins a full season of Justin Verlander; the incumbent talent of Dallas Keuchel, Lance McCullers Jr. and Charlie Morton; and the depth provided by Brad Peacock, Collin McHugh and recently graduated prospect Francis Martes. How the 2018 Astros' starters will stack up against the best rotations of this century remains to be seen, but one question we can answer now is which rotations set the standard.

There are numerous ways to evaluate the quality of a starting rotation. The overall performance of a team's starters is the obvious one, but that can yield some top-heavy results, such as the 2015 Dodgers with Clayton Kershaw and Zack Greinke or the 2002 Red Sox with Pedro Martinez and Derek Lowe, as well as some amorphous assemblages such as the 2017 Dodgers, who had just one pitcher qualify for the ERA title (Kershaw at 175 innings over 27 starts). One could also look for the self-sufficiency of a starting five. For example, the 2003 Mariners used just five starting pitchers, the only team this century not to need a sixth starter. Similarly, the 2005 White Sox, whose top four starters faced all but two batters in their five-game win over the Angels in that year's American League Championship Series, used just six starters during the regular season and received 1,074 regular-season innings from their starters, the most since 2000.

I opted to combine the two. For overall performance, I divided each team's starters' ERA by the league ERA to get a percentage that is effectively ERA+ without the park-adjustment, with 100 representing league average. Except, in this case, the lower the number, the better. I then averaged that figure with the OPS+ against each team's starters (the only park-adjusted stat available in Baseball-Reference's team-level pitching splits). I'll call that the team's OP-ERA score. To ensure the proper concentration of starts among each team's five most frequent starters, I eliminated any team whose top five starters (by games started) started less than 85 percent of its games.

The result is the following list of the five best starting rotations of the 21st century.

5. 2015 Cardinals

Top 5 starters: John Lackey, Michael Wacha, Carlos Martinez, Lance Lynn, Jaime Garcia

81.3 OP-ERA (86 OPS+, 77 ERA%); 89.5 percent of starts by top five

The 2015 Cardinals were one of just three teams this century to have a collective starter's ERA below 3.00, posting a 2.99 mark in 979 2/3 innings. All five of the pitchers listed made at least 20 starts and posted an ERA+ of 116 or better. Only six teams in Major League history have had five pitchers make at least 20 starts with an ERA+ of 116 or better in the same season. Prior to these Cardinals, the last was the 1944 Pirates. The 2017 D-backs have since joined the list, but only two of their starters exceeded 175 innings pitched. By comparison, each of the first four men above threw more than 175 innings for St. Louis in 2015. According to both ERA and WAR, the worst of the Cardinals' five primary starters in 2015 was Michael Wacha, who went 17-7 with a 3.38 ERA (116 ERA+) in 30 starts and was worth three wins above replacement.

4. 2017 Indians

Top 5 starters: Corey Kluber, Carlos Carrasco, Trevor Bauer, Josh Tomlin, Mike Clevinger

80.8 OP-ERA (81 OPS+, 81 ERA%); 85.8 percent of starts by top five

The Indians' starting pitchers stuck out 1,066 batters at a rate of 10.1 per nine innings, both of those marks all-time records for a team's starters. Cleveland used seven starters in 2017, the five listed above plus Danny Salazar and Ryan Merritt. Of those seven, only Tomlin had an ERA+ below league average. Kluber won his second American League Cy Young Award, leading the Majors with a 2.25 ERA, 202 ERA+, 0.87 WHIP and 7.36 strikeout-to-walk ratio. He tied for the MLB lead in complete games (five) and shutouts (three), and tied three others, including Carrasco, for the lead with 18 wins against just four losses. Carrasco added 226 strikeouts in 200 innings with a 4.91 K/BB and 139 ERA+. After opening the season in the rotation, Salazar missed roughly two months due to shoulder and elbow inflammation, but Clevinger and Merritt provided all the depth Cleveland needed by posting a 2.69 ERA in 25 combined starts.

3. 2003 A's

Top 5 starters: Tim Hudson, Barry Zito, Mark Mulder, Ted Lilly, Rich Harden

79.5 OP-ERA (79 OPS+, 80 ERA%); 85.8 percent of starts made by top five

The 2001, '02 and '03 A's all sit very high in my rankings, but the '03 team edges out the other two in my OP-ERA stat. In '01 and '02, the team's No. 4 starter was Cory Lidle, who had his two finest seasons in Oakland, posting 116 OPS+ in 380 innings over those two campaigns. Lilly came over in the three-team Jeff Weaver trade in July 2002 and took over Lidle's spot in '03. Top prospect Rich Harden joined the rotation in July 2003, after veteran John Halama pitched his way into the bullpen. Lilly and Harden were roughly league-average in their combined 44 starts. What made this edition of the A's rotation the best of the Moneyball era was the performance of the Big Three -- Mulder, Hudson and Zito -- all three of whom posted an ERA+ of 135 or better. Though Mulder was limited to 186 2/3 innings by a hip injury, he and Hudson had their best seasons by both ERA+ and bWAR, while Zito had what was arguably his last great season. Hudson and Zito combined for 471 2/3 innings over 69 starts. Together, the three were worth a combined 18.6 bWAR.

2. 2011 Phillies

Top 5 starters: Roy Halladay, Cliff Lee, Cole Hamels, Roy Oswalt, Vance Worley

75.5 OP-ERA (76 OPS+, 75 ERA%); 85.8 percent of starts made by top five

Perhaps the most celebrated rotation of the century only lasted one year. Oswalt left as a free agent after the 2011 season, and Halladay's shoulder gave out in '12. For that one year, however, it lived up to the hype. Halladay, Lee and Hamels finished second, third and fifth in the National League Cy Young Award voting, respectively. Halladay led the NL in ERA+ (163) and complete games (eight) and the Majors in K/BB (6.29). Lee was close behind with a 160 ERA+ and 5.67 K/BB while leading the Majors in shutouts (six). Hamels' 216 innings with a 137 ERA+ and 4.41 K/BB almost seemed pedestrian by comparison. Meanwhile, rookie righty Worley took over the fifth spot in May and went 11-3 with a 3.02 ERA in 21 starts, good for third in the NL Rookie of the Year Award voting behind Craig Kimbrel and Freddie Freeman. Only Oswalt disappointed, missing roughly two months with back issues and proving to be a merely league-average pitcher over 139 innings. Fifth-year righty Kyle Kendrick compensated with a 3.14 ERA in 15 starts.

The 2011 Phillies lead all 21st-century rotations in ERA (2.86), K/BB (4.22) and complete games (18). Their 108 quality starts have been bested only by the '14 Braves (who had 110 in what was the most pitching-friendly season of the century), and their 1,064 2/3 innings pitched rank third behind the '05 White Sox and '03 Yankees.

1. 2016 Cubs

Top 5 starters: Jon Lester, Jake Arrieta, Kyle Hendricks, John Lackey, Jason Hammel

69.6 OP-ERA (68 OPS+, 71 ERA%); 93.8 percent of starts made by top five

The 2011 Phillies may have had more star-power, but the '16 Cubs produced better results, and more of their starts were concentrated among their top five, who started 152 of 162 games in their championship season. These Cubs were one of just three teams this century to have five starters qualify for the ERA title with an ERA+ of 100 or better (along with the '11 Rangers and '05 Cardinals). Better yet, they are the only team since the 1949 Tigers to have five qualified pitchers with an ERA+ of 109 or better. Like the 2011 Phillies, these Cubs placed three pitchers high in the NL Cy Young Award voting, including Lester (19-5, 171 ERA+ in 202 2/3 innings) in second and Kyle Hendricks (0.98 WHIP and a Major League-leading 2.13 ERA and 196 ERA+ in 190 innings) om third. Defending NL Cy Young Award winner Jake Arrieta (18-8, 135 OPS+ in 197 1/3 innings) finished ninth.

The Cubs' starters got a ton of help from their fielders. Collectively, they held opponents to a .254 batting average on balls in play compared to a league average of .301 (by comparison, the 2015 Cardinals' starters all seemed to overachieve at once, but their collective BABIP was just one point below league average). Still, the results are what matter, and no 21st-century team's starters have had a lower collective WHIP (1.07), held opponents to a lower OPS+ or posted an ERA lower relative to league average (2.96 vs. 4.16) than the 2016 Cubs rotation.

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Cliff Corcoran is a Sports on Earth contributor and a regular guest analyst on MLB Network. An editor or contributor to 13 books about baseball, including seven Baseball Prospectus annuals, he spent 10 seasons covering baseball for and has also written for USA Today and SB Nation, among others.