Injustice 2 is quite possibly the best mobile game I have played. Something that is fair and easy to get into but difficult to master. Within 1-1.5 weeks of playing it, I have unlocked 12+ heroes, got some of them to as high as 4K threat level and still upgrading. Game just keeps on giving.
If you like fighting games and want some great graphics and ass busting this is the game to get. It is free and honestly thus far, I have not felt the need to pay to win. Plenty of things to do as well in it. I am spending close to an hour every day punching rounds.