Here is a standout game for the iPhone: GeoDefense. It is one of the many Tower Defense style games, but it is very well executed with an incredible level of detail and smooth animation. It comes at a cost though: the 3GS iPhone gets hot, and the battery drains quickly. The objective of the game is to prevent critters from reaching the end of a path by placing various towers with lasers or canons along the lines of that path. The easy levels are easy, but then one must actually use a strategy to beat the game.