What kind of games do you like?
I like puzzle/strategy games--and have found Trainyard to be exceptional. (There's both a smaller free version and a larger pay version, but the puzzles aren't repeated between the two.) (Note that I've only played it on an iPhone, not iPad, but I have to believe the experiences will be comparable.)
If you're looking for a good summary of some of the best games available recently, in many different genres, check out the link below. Note that the page in that link only shows the "winners" in each category--but clicking on each category name will also show the full list of nominees/honorable mentions, where you'll find many other great ones too.
And be sure to check out the 2009 and 2008 winners too (links to those are in the upper right corner of the page) to see even more. Some of those are now available free of charge...