High Noon

If it is possible for an app to already be a classic, then High Noon would be it. The idea is based on a lot of the gunslinger games that predate it on computers and consoles. But with your iPhone you have to holster and then draw — you actually have to hold the device by your side and draw to start a duel. Then you need to move your iPhone around to target your opponent. This makes it much more challenging than point-and-click game.