You'll probably get more helpful suggestions if you say what sort of games you like. Just a few notable ones to mention off the cuff:
Dead Space - Apple's choice for iPad Game of the Year in 2011
Infinity Blade II - known as one of the flagship iOS games in terms of graphics and visuals
World of Goo
Machinarium - really clever puzzle / challenge game (iPad 2 only I think)
Word Ladder by Sporcle - if you like word games, this is one of the most addictive
Flick Golf Extreme HD - crazy fun - try to get a hole in one with the pin on the deck of an aircraft carrier and you're hitting from inside a helicopter