Fieldrunners

Surely Fieldrunners came a bit ago, it's not new and neither is the concept. But on the iPhone this is the greatest execution of the Tower defence gameplay. As you can see the graphics are really sharp and both towers and troops are fully animated.
The only downside is that, as of now, there's only 2 maps and 4 types of towers, but Subatomic Studios add features with every release, so maybe the situation will get better.

I got really hooked by Fieldrunners, of course it's only tower defense after all, but if you own an iPhone or iPod Touch, you should give it a try. I think the current price is around 3 dollars...