Mostly, it's the must-have Indie darlings, with a sprinkling of AAA fodder.
I've loved:
Rimworld
Yakuza 0
Slay The Spire
Dave the Diver
Dredge
I've found that I enjoy playing offbeat adventure games on the deck more than I did sat at a PC. Detention, Norco, Loretta, To The Moon, that sort of thing is perfect for playing in bed, propped against the pillows.
I'm personally a huge fan of project zomboid, though I had to tweak the controls a little to make it work well.
I'm sure Stardew Valley is a perfect match for the deck, but I've played it too much already.
Pick the genre you like and get started. I prefer the deck for short term bursts so I have all my roguelikes on it. Also bg3 for when the craving sets in.
I mean im a first time owner as well so take it with a grain of salt but I have played and enjoyed cyberpunk, starfield, baldurs gate, and harry potter.
Enter The Gungeon - even if you aren’t great at it, the fact that it launches DIRECTLY into challenging gameplay means it works great on the go. You only need 4 minutes to get into the meat of enter the gungeon and I think in that way it is really elevated by being in mobile format.
Spelunky 2 - a modern classic, same point as above
Monster Train (my pick of the two) or Slay The Spire - ya knowww
Halls Of Torment - if you are into bullet heavens like vampire survivors
Idk hard to give recommendations without knowing more what you are into!
Besides maybe competitive shooters (and you can still try, and do well with practice), and games with many keyboard inputs (Terraria for me), anything you'd play on a normal PC. Not much changed in games I play since I got the Deck.
And games like easier shooters (Borderlands) or mouse reliant games (flash games, in my case) still play well, so it really does seem like it's only those two specific types I mentioned.
I really enjoyed playing through Tunic on my Deck. I imagine Corpse Party (2021) would also be a great Deck game (I played the game before I got mine.)