I picked a Deck up after my son was born. For me, being able to lock the Deck at any point & then resume the game later was the best bit. It meant any game was "baby compatible". I replayed a lot of Xbox 360 and Xbox One era titles.
Bioshock series
Tomb Raider (2013) series
Shadow of Mordor/War
Batman Arkham series
The Elder Scrolls
Fallout (up to & including 4/London)
The Witcher series
Red Dead Redemption 1&2
Halo: MCC
Assassin's Creed (up to Odyssey)
Half Life 2 & Black Mesa
Mass Effect series
Portal 1 & 2
The Outer Worlds
Forza Horizon 4 & 5
As a general rule of thumb, if it ran well on an OG Xbox One, it'll run well on Deck.
Aside from the above, I've got all of the usual Deck staples: Stardew Valley, Vampire Survivors, Balatro, Dave the Diver, Hades, Rimworld, Factorio, Hollow Knight, Celeste. It's a great bit of kit for these less demanding indie games.
I've largely shied away from more recent games on Deck as when I've tried them I've found they mostly don't run well. I did play through much of Kingdom Come Deliverance 2 on Deck earlier in the year though. That ran surprisingly well.