super hexagon
EDIT: heres some gameplay footage for those who havent seen it:
http://www.youtube.com/watch?v=5mDjFdetU28
Basically, you need to last 60 seconds to beat whatever level you're on. The whole game only has 6 different levels, but ive somehow managed to dumb anywhere from 10-20 hours into it so far, and ive only beaten 2 of the 6 levels.
I hated it at first, and would often die really quick even on the first level, but its such a quick game and easy to pick up that I kept coming back, and loving it.
The closest thing I can compare it to is hotline miami. (in terms of getting killed over and over, but just jumping back in every time)