Crossy Road
I have to say, if you value your free time, do not download Crossy Road. Part Frogger, part Flappy Bird and 100% time suck, this disarmingly charming mobile game is sadistic in it's addictive design.
The rewards come easily at first, then longer, and of course you can buy the sprites. Some don't do much, but others reward with a little animation, funny sound or action. The mad wizard sets trees on fire, the celebrity is hounded by paparazzi, the unihorse leaves a rainbow trail, the leprechaun has a demented giggle and so on. The only redeeming level is the Pac Man one!
But there's another reason to avoid Crossy Road - on Android at least, it's a huge resource hog and uses a lot of data. When I tried to play it with data disabled, it simply crashed. I ended up uninstalling it on my phone and reinstalling it on my tablet, which stays at home. Probably a good thing!