Here's the Indians 1-4 pitching rotation (as of June 6th) with their stats:
Corey Kluber: 9-2, 1.96 ERA, 0.85 WHIP, 95 K
Trevor Bauer: 4-4, 2.77 ERA, 1.16 WHIP, 97 K
Mike Clevinger: 4-2, 3.36 ERA, 1.22 WHIP, 64 K
Carlos Carrasco: 7-4, 4.23 ERA, 1.19 WHIP, 83 K
Pretty damn solid, although Carrasco's had a few bad games that make his counting numbers not look that great. Lower WHIP and more strikeouts than Clevinger, but almost a run worse in ERA. Carrasco is good and will likely settle into around a 3.20-3.40 ERA this season for all of those guys who are into that stat.
Here's a few rotations that I found comparable:
Houston
Justin Verlander: 7-2, 1.24 ERA, 0.72 WHIP, 104 K
Gerrit Cole: 7-1, 2.16 ERA, 0.84 WHIP, 124 K
Charlie Morton: 7-1, 2.84 ERA, 1.08 WHIP, 92 K
Lance McCullers: 7-3, 3.94 ERA, 1.19 WHIP, 76 K
Dallas Keuchel: 3-8, 4.13 ERA, 1.22 WHIP, 63 K
The best rotation in baseball right now in my mind. Their #5 guy this season is a former Cy Young award winner. McCullers has come back down to earth a bit, and I still expect Verlander and Cole to revert a bit to their norm, but both could easily be top-5 guys in the Cy Young race at the end of the season. I'll give this current Houston rotation the edge over Cleveland, especially since they have five consistent starters to our four.
LA Angels
Shohei Ohtani: 4-1, 3.10 ERA, 1.13 WHIP, 61 K
Andrew Heaney: 3-4, 3.12 ERA, 1.10 WHIP, 58 K
Garrett Richards: 4-4, 3.25 ERA, 1.23 WHIP, 69 K
Tyler Skaggs: 4-4, 3.27 ERA, 1.27 WHIP, 70 K
A bit of a surprising mention here, but the Angels' top-4 have been right up there with the best in the American league so far. I do think it's mostly an aberration and advanced stats say that they're overachieving for the most part, but I do think it's been interesting. I'd expect Richards and Heaney's numbers to tail down as the season goes on and Ohtani is currently on the DL. Their 5th starter, Tropeano, is nothing to write home about either.
Washington
Max Scherzer: 10-1, 1.95 ERA, 0.83 WHIP, 133 K
Gio Gonzalez: 6-2, 2.27 ERA, 1.23 WHIP, 72 K
Jeremy Hellickson: 2-0, 2.28 ERA, 0.92 WHIP, 34 K
Stephen Strasburg: 6-5, 3.20 ERA, 1.05 WHIP, 93 K
Tanner Roark: 3-6, 3.56 ERA, 1.09 WHIP, 65 K
In my mind, the best rotation in the National League is led by the best pitcher in baseball in Scherzer. Gonzalez, Strasburg, and Roark make up a helluva 2-4 in the rotation, and Hellickson, currently on the DL, has started out extremely strong. I'd place them ahead of Cleveland but behind Houston.
St. Louis
Carlos Martinez: 3-2, 1.83 ERA, 1.16 WHIP, 52 K
Miles Mikolas: 7-1, 2.27 ERA, 0.95 WHIP, 58 K
Michael Wacha: 7-1, 2.41 ERA, 1.09 WHIP, 61 K
Jack Flaherty: 2-2, 3.20 ERA, 1.16 WHIP, 42 K
Luke Weaver: 3-5, 4.12 ERA, 1.23 WHIP, 57 K
The best rotation in the NL Central is also very young, with none of these guys over 30 years old. Wainwright looks about done, so this should be their guys going the rest of the way. Martinez, Mikolas, and Wacha all have #1-2 guy stuff, but the jury is still out on Flaherty and Weaver who have a bit to prove.
Other Honorable Mentions
Boston (Sale, Price, Porcello, Rodriguez) - no current 5th guy with Pomeranz struggling and on the DL. Sale is pitching well, Price continues to face his decline a bit, and Porcello and Rodriguez are good middle of the rotation guys. Cleveland is definitely better.
Philadelphia (Nola, Arrieta, Pivetta, Velasquez, Eflin) - kind of an interesting group of guys here. They've got a lot of youth led by their ace Aaron Nola paired with Jake Arrieta. Worst ERA of this bunch is Velasquez with a 3.82, who also has the worst WHIP at 1.28. Advanced stats, like the Angels, say this group is overachieving a bit and should regress, but they've been damn solid to start the year.
LA Dodgers (Kershaw, Wood, Ryu, Hill, Maeda, Buechler) - four of these six guys are on the DL right now. They have a very high ceiling, but if they can't stay healthy, it won't matter.
If I had to, I'd currently rate the rotations in baseball:
1) Houston
2) Washington
3) St. Louis
4) Cleveland
5) LA Angels
6) Philadelphia
7) Boston
Healthy Dodgers playing up to par are probably top-4. And just for reference, the Yankees rotation sans Severino has been average at best. Tanaka and Gray's ERA/WHIP are 4.79/1.14 and 4.81/1.46 respectively. Sabathia has been pretty solid but is 57 years old. Montgomery is on the DL.