Rocco, Cabaretti Caterer is a card that will probably excite Commander fans the most because they can now essentially play any nonlegendary creature as their commander if they are willing to pay three more mana for it. The games usually take fairly long so you will be able to do this more times over the course of the game.
In Standard, Rocco will probably find a home in some kind of midrange or ramp deck where you’ll be able to grab some utility creatures with it. Three extra mana isn’t that much to give your deck a versatile answer for most situations and with all the Triomes, Pathways and slowlands, you shouldn’t have an issue playing three or even more colors.
Having an extra body in play is now even more relevant thanks to the new casualty ability as well.
But it might be even better in Modern, where the card pool is much deeper.
There are many cards you can get with his trigger without any additional investment. I’m not sure how useful would be getting a free Memnite or Ornithopter in a Naya deck, but how about a Dryad Arbor? Ramp effects are always good and they don’t usually come for free.
Kiki-Chord style decks get a new tutor effect and this time even on legs, which means you can return it with Teferi, Time Raveler into your hand and do it again.
All the way from Dryad Arbor to tutoring Craterhoof Behemoth for the kill in a deck like Elves, I’m sure Rocco will find plenty of use in Constructed formats. But Commander has to be the real winner. What will you find with him?