My go-tos at the moment are:

• Oldschool Runescape - wooooooooo
• Danmaku Death - old, free, no in-app purchases or ads bullet hell game from 2014, but it’s delisted from the play store…

Other games I’ve enjoyed

• Super Auto Pets. Base game is free with no ads, but there are cosmetics and expansion packs.
• Pixel Dungeon and its variants. Free roguelike with no ads and no in app purchases.
• Magikarp Jump. Fun little incremental game where you make magikarp jump. Has in-app purchases.