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.
Slay the Spire is excellent on mobile, it’s a roguelike deck builder.
Slay the Spire is crazy fun. And literally infinitely replayable
Magic the Gathering: Arena. The mechanics for getting gold, packs, and cards uses semi-toxic dailies/weeklies, but it’s do-able to play free2play.
Cytus II, it’s a great rhythm game with a cool storyline
Definitely the mobile version of Stardew Valley
May be the commercial one but actually Candy crush is a nice one
I work at a gaming company - Candy crush is known for employing the same tactics that keep people sitting at slot machines for 8 hours a day, and every damn PM wants to copy those patterns.
