My favourite game on my iPhone at the moment is geoDefense, a tower defense game. Simple gameplay, nice graphics, addictive game overall.

Welcome Screen and Level Selection:

Level Start & Game View:

Game View and End of a Game:

End of a Game and About Screen:

While the game isn't free, it's only $0.99. Best of all, there's also a free light version available in the App Store for testing!