What people are missing when talking about the cubs rotation is the depth. Here's where the various pitchers finished last season
2 Jake Arrieta - 7.3 fWAR
13 Jon Lester - 5.0 fWAR
24 John Lackey - 3.6 fWAR
27 Kyle Hendricks - 3.4 fWAR
49 Jason Hammel - 2.4 fWAR
You then talk about their bullpen where Travis Wood had 1.3 fWAR although not enough to qualify as a starter. Like wise Adam Warren had 2.2 fWAR but didn't qualify. And to round things out you have Cahill who had promise at times, Edwards who is obviously a well regarded prospect. So, the cubs have 4 of the top 30 pitchers from last season and a ton of depth.
Where I would argue there is a difference is the cubs probably lack some top end but in terms of depth I'd imagine they are pretty unrivaled. Last season, the Royals had 2 starters over 2 fWAR. The cards had the best team in baseball and had Lackey(3.6), Martinez(3.4), Lynn(3.1), Garcia(2.8), and Wacha(2.3). You're swapping Lynn with Leake and Lackey with Wainwright. The mets had DeGrom(5.2), Harvey(4.4), Syndergaard(3.1), Colon(2.5) and no 4th guy over 1. If you assume Wheeler is in there with his 2014 form it was 2.5 fWAR though I'd doubt Colon keeps up that pace. They also still have Matz but who knows what he is yet. LA lost grienke and is rebuilding it's staff some. I mean i could go full out here but that covers the "best" teams