A lot of really good older games have been ported to mobile and work fantastically. Knights of the Old Republic, Neverwinter Nights, and Baldur’s Gate just to name a few. Hell, you can play Baldur’s Gate multiplayer, cross-platform with PC players even! There’s also fo2.exe on the Play Store, which lets you play Fallout 1 & 2 on mobile provided you have access to the files from the PC versions. Oh, and Doom ports galore, of course.

As far as mobile-only or mobile-specific games, my favorite is probably Caves, a surprisingly well done and completely free roguelike.

Definitely the mobile version of Stardew Valley

Slay the Spire is excellent on mobile, it’s a roguelike deck builder.

RCT classic is pretty good. Not as good as OpenRCT but still quite good.

Slay The Spire is also very good, though the mobile version is a bit bigger than the PC game