Beatrider, World of Tunes and DDR-S are a few of those apps that you'll always keep on your iDevice, even though you might only play a few minutes at your time, thus having the highest replayability.
Call of Duty: World at War: Zombies. Best $10 to date. It just keeps you coming back over and over again, and I'm only playing single player with 1 map. There's more content planned to come and with online multiplayer it should just be insane
