Forget the big-name games that are always at the top of the App Store -- when I got my iPhone, Kingdom Rush was the first thing I downloaded. This is tower defense done right, especially in terms of challenge, and for the rest of the day, the game is free to download. Go get it!
Those of you who take the time to upvote on Steam Greenlight might also be interested to know that Ironhide Studios has an expanded version of Kingdom Rush up on the service. It's supposed to offer a new undead campaign and seemingly more of everything found in the core game. And no, it's not a port of the Flash original. I'd be down.