I would say at the moment, Fastar! But overall, i would say Slotz Racer. I havent beaten it yet, and the progress has been wiped so many times (due to rejailbeaks and updating) yet i somehow come right back to it to try and finish it.
Tilt to Live, absolutely. The awards combined with the high-score gameplay means I keep on playing it.
Currently, I can't put down Helsing's Fire or Phoenix Wright, but as far as long-term appeal goes, you can't beat Tilt to Live. The classic mode alone never gets old, and with 3 (well, 2 and a half) other modes to choose from, it makes for the best arcade game on the App Store. Period.
Bit of a different one from me, but in all honesty it's probably Words with Friends, it must be the game I've spent most hours playing in the last 12 months. And it gets your brain thinking as well Probably would be Tilt to Live otherwise, but I'm so crap at it
Right now, Fastar! But Tilt To Live is up there as well, though Classic mode is so boring, gotta play Code Red mode
Action Hero and Lumines are the ones i play the most, by far. While i enjoy games like Fruit Ninja, Tilt to Live or Angry Birds, i don't play them over and over like Lumines and Hero.