The Skullgirls Mobile game is pretty fun. Not as deep and fun as the original game of course, but they simplified it in a great way and you get to enjoy the wonderful spritework/animation on the go, so I definitely recommend it.
and +1 to DBZ Dokkan Battle. I never spent any money, but damn the amount of time I spent sitting on the toilet felt like it doubled because I'd get stuck playing that game and grinding for rainbow URs...