Depends on how you define strength.
Overall, I think most commanders are stronger than stetmann’s infestors, since for less ramp up time you can get things like 14 well stacked ascendants, or 8 carriers + gate support from Fenix, or mass spider mine from raynor, or a fully stacked up Tychus with buddies, or 3 Brutalisks + 3 leviathans, or 3 Hercules full of tanks, or 15 Omega Networks (which can spawn WAY more hp)
Meanwhile, in the time it takes for him to make those infestors, the map is more than half cleared by Artanis, or Zagara, or Fenix, or Bio Raynor, or on a few maps even Kerrigan hero solo.
And meanwhile, the only armies slower or more cumbersome than Stetmann’s infestor glob are mass Thor (unless you use hercules) or Abathur’s swarm hosts (which are bad) and ascendants (which are significantly better than infestors and have deathfleet and overcharge to cover them). And yes, that’s accounting for stetzones, if you have speed boost on then that means you won’t have a lot of energy when you get there.
However, Stetmann’s corruptors are possibly the best anti-air in the game (tough call against scourge), his lurkers are incredibly good, having burrow charge on LURKERS, and being able to cast their focused strike for EVEN MOAR AoE. Further, his hydra/ling/bane is a better early-midgame than most commanders are capable (not all, but most) and transition rather smoothly to corruptor/lurker. And then he has his ultras, which are very specialized but good at what they do.
And to clarify, yes, I am saying that his infestors are not his best build, I think that they are only his best build on defensive maps where you can just turtle in the purple stetzone and spawn faster than the units die. There, it’s his best build. But otherwise, I don’t think so.