@paulasadoorian

@steamdeckhq just posted an article about the most played games on #SteamDeck that could be informative.
Valve Releases List of Best Steam Deck Games of 2023

That said, it does really well for slightly older games and tons of indies that are in early access and not yet certified.
Check out the ProtonDB website to see what games that might not be certified actually work just fine.

I'd personally recommend
#VampireSurvivors, #SoulstoneSurvivors, #Brotato. Just about every game in the survivor / bullet heaven genre plays fine since the controls are normally so simple.
#DeadCells, #RogueLegacy2, #TrinityFusion. There are tons of these metroidvania-rogueli[tk]e games that work great.
#BaldursGate3 has a wonky controller interface, but that hasn't stopped me from putting in over 200 hours on my deck. The #DivnityOriginalSinEnhancedEdition interface is much better even though it's the same developer. #DivinityOriginalSin2 works well, too.
If you like #ARPG games, I've played a few such as #GrimDawn (which works but does not have official support), #VictorVran. I tried #DungeonSiegeIII which was a disaster (it also didn't play well on the deck).
#ElderScrollsOnline works surprisingly well once you get it fully installed and configured. It's the only MMO I can play on my deck.
I put about 20 hours into #Tyranny when I first bought my deck in late 2022.

Edit:
#Warframe should be another MMO that works on the deck, but I haven't played it in a few years so I haven't tried.