In my opinion, Halls of Torment/Vampire Survivors/Brotato are staples for any Steam Deck users.

I know you asked for a #1 recommendation, but these three games are kind of similar, depending on your vibe.

PARANORMASIGHT: The Seven Mysteries of Honjo! Surprsingly comfy to play on the deck.

Congratulations! I hope you can catch up on that indie games backlog you’ve never played XD (speaking from experience)