Fun for me is always classic mobile puzzle games.
I have searched far and wide for a good solitaire app after my go-to one introduced a subscription service... in my solitaire game. sometime in the last few months I finally found one, just called Solitaire developed by Still57. no weird gimmicks restraining how much you can play or anything like that.
Same for nonograms/picross, most apps tend to have a three lives then out gimmick, or something or other, and waaay too many ads. Nonograms CrossMe developed by Dynamix Software is simple and straight forward, just the puzzles. They also have a pretty good sudoku app, another genre of game that's mostly filled with gimmicks and ads. again, their app is perfectly basic.
Any Flow game is a good game. I've been making my way trying to 100% them, so far I've done the original and Hexes.