iPad gaming isn't very dynamic, most games come from iphones ported/adapted to iPad and the few available only on iPad most aren't total top pick.
Here a very short list of game released on iPad and not on iPhone, I won't stick to action games only:
Knight Defense: It's a very special mix between Tower Defense, and action elements. As levels difficulty increase it becomes more an action game with some TD elements. Great game, for me, the best I know of iPad only games. There's a demo you could try.
Galcon Fusion: Do you see Galcon Labs for iphone? Then Galcon Fusion is a lot better and on iPad but not iPhone.
Mirror's Edge for iPad: A sort of Canabalt more sophisticated with more different actions, much better graphics and a story. A difficult game.
Enchanted Forest: Not the best Brick Breaker, but very special Brick Breaker, focus more on precision and concentration with complex levels with many intermediate goals. Even if you don't like Tilt, you should try it with Tilt commands, it makes it shines.
Splash Duel: A special game mixing pong and brick breaker. There's an AI for single player game, you can set AI strength from 1% to 100%. Single player is really fun, but a game that really shine when played at two.
Shot Shot Shoot: A very special action game, rude in single player but worth it. But it seems shine for playing against someone, from all feedback I read.
My Kingdom for the Princess: It's a different time resource management. Instead of being in a closed area as most game like that, each level is a map with road and elements and you have special goals. Very good game, quite more than the casual time resource management game.
MagniFusion: A sort of action puzzle, it has similarities with Eliss but plays quite differently. When levels start be less easy it's a good game.
Eden's Quest: It's a mix between a game like Professor Layton and moderns Find the item adventure games. But it's a solid good game but far to be as good than some other similar games ported from iphones.
Now I'll be clear, a list of ipad games ported from iphone or the reverse will make a stronger selection. In a next post I'll list some of the best with a gameplay somehow different from the iphone version.