Temple Run is a game I play a lot.
Mega Jump is also pretty good I find, I also find it a lot easier to play the iPad even though it's not the full screen.
Diamond Dash is good if you have a Facebook account, good if you just want to just waste a few minutes at a time.
Just downloaded Jukebeat today, the US version of Jubeat by Bemani (Konami's music game division). If you like the other Bemani games (Beatmania, DDR, Para Para, etc) or games like it (Guitar Hero, etc), then this would be a good choice. My only gripe is the songs are super expensive ($1 each).