So many, it's really impressive. Vampire Survivors to Elden Ring (with coop mod and Discord running in the background on SD), to No Man's Sky, to Diablo 4 (through installed battle.net added to Steam as a Non-Steam game on the SD) all run great.
Also, if you have a good pc you can run the game there and stream it to the SD, extends run time on battery by quite a lot.
Tunic is a blast on steam deck, I highly recommend it. I've also been playing Vampire Survivors, Dishonored, emulated Nintendo games: Breath of the Wild (wii U), Super Mario Bros (switch) , Advance wars (ds).
Basically anything that can be played with a controller works well, and some of what needs a mouse and keyboard is also good.
My current favorites are gamecube games (Tony Hawk Games and 007 Nightfire) and Forza Horizon 5. Played quite a bit of minecraft aswell, but I find it hard to be efficient with a controller.