Slay the Spire is excellent on mobile, it's a roguelike deck builder.
Seconding this, it's one of the very very few mobile games that hold up to the standards of other platforms (probably because it's on those other platforms as well). It's not just a good mobile game, it's an incredible game period.
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.
Also an honorable mention goes to Organ Trail, a zombie-themed take on Oregon Trail that is, frankly, just a flatly better version of the game in almost all regards. The only reason it's not my favorite is because the controls on mobile feel awkward compared to the PC version. Still fun though.
I am playing a lot of slice and dice right now. Rogue-like dice rolling game. Available for PC and mobile. Free demo available. If you buy it on Itch.io you get a code for activating your mobile download as well. Absolutely worth the 7 dollars I spent on it.
