My favourites are Hitman GO and Superbrothers: Sword & Sworcery EP.
Hitman GO is just a great little puzzle game that's easy to waste time with. The whole GO series is great too, but Lara Croft GO and Deus Ex GO are a bit more involved story-wise so they're harder to chill out to. Hitman is the purest experience, with everything looking like a mini board game.
Sword & Sworcery is a beautiful and surreal pixel art adventure game with a great soundtrack. I actually haven't played very much of it, but I can't help but recommend it because of how high quality it is. It's more like a full game than a mobile game (probably why it was also ported to PC and consoles), so it takes a lot more time investment.