Starting rotations are typically the basis for one of the best, and arguably most meaningful, baseball debates. But usually we only have those conversations going into and coming out of the season.
Here at the mathematical midway point of 2016, it's worth checking in on which clubs have met or exceeded expectations in this area -- and, ergo, which ones could be particularly dangerous come October.
All stats are through Sunday's play.
1. Cubs
ERA rank (among 30 clubs): 1
FanGraphs-adjusted ERA: 1
Fielding Independent Pitching: 4
WHIP: 1
AVG. against: 1
K/BB: 4

Going into the year, many of us, Cubs' execs included, had some questions about the back end of this rotation, which is why the Cubs did investigate the trade market for young, controllable starting pitching. And if you were a particularly worrisome sort, you were free to fret about the innings effect on Jake Arrieta, the age effect on John Lackey and the bone chip floating in Jon Lester's throwing elbow.
So far, for the most part, those questions have proven unwarranted. In many meaningful statistical categories (including ERA, where the Cubs entered the week with baseball's best mark by nearly half a run), this rotation has been tops in the game, and that's partially due to it being the healthiest rotation in the game. None of the Cubs' starters has missed a start, to the point the club stretched out reliever Adam Warren, who gets the ball Wednesday, to lengthen its rotation to a six-man unit and dole out extra rest to the starters.
None of which is to say there isn't lingering concern moving forward. Arrieta's command hasn't been as sharp as of late, and Lester and Jason Hammel both got battered over the weekend. Simply as a product of pitcher injuries being so ubiquitous in baseball, perhaps the Cubs are "due" on that front. Don't rule out the Cubs using their ample trade chips to acquire some starting insurance this summer.
Again, though, all we can accurately analyze is what has already happened, and the Cubs have received superior starting pitching in the first half.
2. Mets
ERA: 2
Adjusted ERA: 4
FIP: 1
WHIP: 6
AVG: 12
K/BB: 1

This was a no-doubt No. 1 going into the season, but Matt Harvey had a clunky start and he is trying to grind his numbers back to an elite level, but now injuries -- or, at least, the threat of injuries -- are a consideration in Queens. This is the scariest rotation in baseball, both from the standpoint of what it could accomplish again on the October stage, and also from the standpoint of how quickly it could come undone if the elbows don't cooperate. Noah Syndergaard and Steven Matz are both pitching with bone spurs. Syndergaard called his "pretty significant," and Matz's issue, which will require surgery at season's end, has forced him to significantly scale back his slider usage.
But the bottom line is Syndergaard, Jacob deGrom and the unstoppable Bartolo Colon all rank in the top 25 among qualified starters on the adjusted ERA leaderboard. Matz was ranked 41 in that area. If Zack Wheeler, who seems to have dodged an elbow scare of his own during his Tommy John rehab, can come back healthy and effective in the second half, he'll allow the Mets to be careful with the innings of the young guys.
All the potential elbow issues are a little alarming here, but we usually have no idea what physical issues guys are working through (Lester pitched with his bone spur for years before fans and media learned about it). Ultimately, if this group stays healthy, it's quite clearly one of the best in baseball -- if not the best -- especially with Harvey (Monday's result aside) trending back in the right direction in recent weeks.
3. Indians
ERA: 5
Adjusted ERA: 5
FIP: 7
WHIP: 3
AVG: 4
K/BB: 7

The arrow is pointing upward here. This is certainly the best starting staff in the American League, and it could possibly have an argument as the best in baseball when you look at recent strides and account for the difference in leagues.
We knew the top three in this rotation (Corey Kluber, Carlos Carrasco, Danny Salazar) was very strong going into the year, but the back end was a question mark. Trevor Bauer refining his repertoire and beginning to throw effective strikes more consistently was the development that has allowed this rotation to untap its true potential. But it has also grown because Salazar (whose adjusted ERA going into his Independence Day outing was second only to that of Clayton Kershaw) has legitimately become one of the best starters in baseball with a changeup that FanGraphs ranks as one of the most effective weapons in the game right now.
Among starters with at least 60 innings this season, Salazar, Carrasco and Bauer all rank in the top 25 in FanGraphs-adjusted ERA. Fifth starter Josh Tomlin and Kluber are both in the top 52. Carrasco missed more than a month with a hamstring issue, but his early June return brought this group back up to full speed, and the Indians could be zipping along toward an AL Central title. This rotation could be a weapon come October. It's also a rotation that costs the Indians less than $14 million this year, roughly the equivalent of what the Orioles are paying Ubaldo Jimenez.
4. Nationals
ERA: 3
Adjusted ERA: 3
FIP: 3
WHIP: 4
AVG: 3
K/BB: 3

The weight-room mishap that forced Stephen Strasburg to spend some time on the DL was a setback, but thankfully that issue did not affect his arm, and Strasburg returned in rousing fashion Sunday. He was unhittable in 6 2/3 innings against the Reds. His absence also allowed us to get our first look at top prospect Lucas Giolito, who looks like he's going to be a monster one day, and he will probably stick in the rotation with Joe Ross now out with a shoulder issue.
Strasburg, Tanner Roark, Max Scherzer and Joe Ross are all in the top 36 in the Majors in adjusted ERA, meaning that putting the Nats fourth on this list might be doing them a disservice.
But Gio Gonzalez's struggles this season have been an issue for this group, and Scherzer can look like the most dominant pitcher in baseball in one start (especially his 20-strikeout effort against the Tigers) and pretty pedestrian the next. Still, the steps forward for Roark and Ross (pre-injury) have taken a rotation that looked pretty decent on paper -- even with Jordan Zimmermann departing in free agency -- and made it legitimately great.
5. Rangers
ERA: 7
Adjusted ERA: 2
FIP: 16
WHIP: 11
AVG: 8
K/BB: 21

Look, I'm going to level with you. This is a top-five list, because we are a society that tends to be oriented around lists of five or 10. But I think there is significant separation between the first four teams mentioned here and whoever goes fifth.
And frankly, you could do a lot of different things with this spot.
You could make a case for the Blue Jays, who have gotten strong years from Marco Estrada, Aaron Sanchez and J.A. Happ, despite Marcus Stroman taking a big step backward. You could make a case for the Giants on the basis of what Madison Bumgarner and Johnny Cueto have done, but Jeff Samardzija has an ERA of 6.53 over his last seven starts, Jake Peavy has an ERA north of 5 and Matt Cain is hurt again. Anyway, if we're going to hand out this ranking on the might of a rotation's top two, you could give it to the White Sox with Chris Sale and Jose Quintana. Or heck, give it to the Dodgers on the basis of having the best pitcher on the planet (even if he is out with a back injury). Before he went on the DL, Kershaw alone had propelled the injury riddled Dodgers unit to the third-highest collective Wins Above Replacement mark in baseball. That's pretty crazy.
Personally, I'm rolling with the Rangers here as a reward for them catching a lot of us off guard with just how good their first-half rotation was, sans Yu Darvish (who only made three starts before going back on the DL). Cole Hamels has, predictably, been very good for them, but Martin Perez also took a huge step forward. Colby Lewis was having a brilliant year before he went on the DL with a lat strain, and A.J. Griffin, despite a DL stint of his own, has been a great comeback story.
This is admittedly a generous ranking, especially given all the injury hits of late, but it's one made with the assumption that the good numbers posted in the first half can be improved upon with Darvish due back soon.