Cube Runner - it's been on there since September, and it remains the king of pick up and play games Edge - freakin' awesome Sway - 'nuff said
Well, it was for me. I was a little skeptical at first because of the five levels, then I got the lite version and fell in love with it, bought it and loved that, played it here and there, lost my iPT, got a new iPT, got addicted to the game and played it an hour or so every day for a week or two, and now I play it on occasion. True story. I don't know how others would feel about it, but I think it's awesome. Excellent use of tilt and touch, good pick up and play game, good music, and fun game modes with item placement specified for each one. I got my $8 out of it.
The game I've spent the most playing in total time is The Quest (if you also include the expansion game). A close second would be Snail Mail. Snail Mail is an awesome game that is heaps of fun.
Defender Chronicles by a long shot for me, followed by Real Racing, Worms and Sim City. I think one of the key reasons I played Defender Chronicles and Real Racing so much is because the developers are very active in releasing new content in updates.
Hard to say =P I've played ShapeShape for 12 hours, according to the game clock, so I'll go with that.
In the past week or over my entire iPhone life? In the past week it has been Star Defense (Trying to get all achievements) Overall, I think BeatRider, Tap Tap Revenge 2 and Orions: Legend of Wizards