The Master Chief collection. Runs great, and has both co-op and competitive multiplayer. Not on sale ATM but I bought it for $10 not too long ago.
Curveball suggestion: Phantasy Star Online Blue Burst. I loved PSO back in the day and it’s amazing to me that I can install it on the Deck through Lutris.
Recommendation for multiplayer games
What are some good multiplayer games to play on the steam deck? They can be online or offline games.