Fun, free, and heavily customizable. If you are a gamer, even in the casual sense, this is the app for you. 

*Edit* (2.1.2020) : One problem to note is that there doesn't seem to be an easy way to renew overdue tasks. By which I mean in other apps I can usually refresh them all with one click so that the tasks are due today. Would love an option for that, since otherwise I have to go into each task and set it to be due today. TT^TT