In baseball, pitching can be everything, so the teams with the most depth are in good shape. This is especially true when pitching injuries are so high. Teams like the Atlanta Braves and New York Yankees are built to withstand the loss of pitchers like Spencer Strider or Gerrit Cole, respectively. And the reason is their pitching depth.
Here are the five MLB teams with the best depth up front for pitchers this season.

MLB pitching rotations with the best depth
#5) Toronto Blue Jays

The one thing the Toronto Blue Jays have going for them in 2024 is a strong rotation. Kevin Gausman has been decent and a Cy Young-caliber performer in recent years. Yusei Kikuchi is having a career year. Jose Berrios is also in good form. Chris Bassitt is a solid pitcher, and if Alek Manoah can regain his 2022 form, the starting rotation will be much better.
A 4.29 team ERA is inflated by a disappointing bullpen, but the depth in the rotation is great.
Team projected fWAR (via Fangraphs): 8.5
#4) Seattle Mariners

The Seattle Mariners do not have a deep lineup capable of putting up tons of runs in an instant, but they don't need to. Their pitching staff is excellent. Luis Castillo hasn't been as dominant as he has in the past this year, but he's still leading one of baseball's best rotations.
Bryce Miller, George Kirby, Logan Gilbert, and Bryan Woo are all quite good. Their 3.58 team ERA only hints at how strong this starting rotation is.
Team projected fWAR: 8.9
#3) Minnesota Twins

The Minnesota Twins have been surprisingly good this year, and it's been because of a dominant rotation. Pablo Lopez is a legitimate award-winning candidate this season, and Joe Ryan has also been excellent. Bailey Ober and Chris Paddack have been reliable back-end starters as well.
Overall, the Twins boast a deep rotation that surprisingly doesn't have as many big names as the production might suggest. The Twins have a 4.18 ERA as a team, but the rotation has been otherworldly.
Team projected fWAR: 9.2
#2) Los Angeles Dodgers

It shouldn't be all that surprising that the Los Angeles Dodgers have one of the best rotations in baseball right now. Tyler Glasnow and Yoshinobu Yamamoto would both be the aces of a lot of other teams' staffs.
To add Walker Buehler, an All-Star caliber pitcher, Bobby Miller, Gavin Stone, Clayton Kershaw, and others to the list is just unfair. They don't even have Shohei Ohtani pitching in 2024, and they're still among the league's deepest rotations. The Dodgers have a 3.29 ERA as a team.
Team projected fWAR: 9.3
#1) Philadelphia Phillies

The Philadelphia Phillies have three aces in their rotation. Zack Wheeler is a perennial Cy Young candidate. Aaron Nola has been a dependable veteran for a very long time. Right now, neither are performing at the level of Ranger Suarez, who is putting together a terrific season.
Those three, with Taijuan Walker and others, form the best rotation in baseball right now. Their team ERA of 3.17 is backed by an exceptional starting five.