A game from Ketchapp called Stack has lately been a small obsession of mine. It's a really simple game involving moving slabs of colors which you have to line up as closely as possible to the slab below it. Each misalignment means a loss in surface area to stack on top of and the game continues until you miss the stack entirely.
The replay value of this game is pretty high in my opinion. There's no storyline and the only goal is to beat your high score. Utterly simple and beautiful. You can play this anywhere, anytime, and kill as much time as you need.