Gravity Guy is a fun one. It doesn't really do anything we haven't seen before -- the basic idea is a running-style game where you simply tap the screen to switch gravity (very similar to the terrific indie game VVVVVV). You can play it right now online as a Flash game, and you only need to get into it for a second before you see the whole premise.
But what's really sent this one to the top of the App Store charts is all the extras and bonus modes that come with it. Not only is there a full story mode to play through, there are also practice and endless modes to play, and even a local or online multiplayer mode. There are Game Center leaderboards and achievements, and both the graphics and music look and sound terrific.
So yes, this is just a Flash game port (and a simple one at that), but it's a Flash game port done very right. Plus, as of this writing, Gravity Guy is on sale today for the low price of absolutely free. Grab it and check it out if it seems like your thing.