So far I have played Trism more than any other game. It really is great.
And it's easy to save your progress so you can play for just a few minutes here or there.
I second this. No matter what new games I download, I always go back to Trism. So many pros...
- Listen to your own music while playing
 - Multiple game modes (timed, puzzle, level/highscore)
 - Simple gameplay supplemented with obstacles (bombs & locks) that raise difficulty.
 - Achievements (has anyone cleared all of these?)
 - Graphics are totally polished
 - And finally, there's a forum for the game and the developer seems very responsive.