Racing games have always been popular -- even non-drivers seem to get a slight adrenaline rush from a dose of speed. Back in 2008, we played Fastlane Street Racing, which won a lot of praise. It's still pretty good, and costs just 99-cents ($3.99 on launch).
Most recently I've been playing EA's HotPursuit, but Polarbit's Raging Thunder 2 (above) is also grabbing eyeballs. Just look at the evolution in terms of transient objects and more inside these games.
Fieldrunners has always held the throne of being the ultimate tower defense title, winning critical acclaim for its art and its intensive gameplay. Gamers also loved that its interface was custom-made for the iPhone.
Tower defense gamers recently began talking about StickWars. This has been on the top sellers list since release and puts players in position to defent their castle against an invading stickman army.
First person action games have always been much-loved on any platform. Three years ago we'd play Hero of Sparta, at its time one of the most impressive hack and slay titles for the iPhone.