There are really so many good games, that its hard to distill it down to a handful of suggestions..
here goes though..
Defense Games - Really well suited for the touch screen
Sentinel 3
Kingdom Rush
Unstoppable Gorg
Anomaly HD
iBomber Pacific
Driving Games - those that do a good job of using the touch screen
Real Racing 2
Reckless Racing 1,2
Smash Cops
Death Rally
Sports Games
NBA Jam
Backbreaker
Great Adventure Titles
Monkey Island 1,2 (these translated really well to the iPad)
Sword & Sworcery
Walking Mars
Retro and Arcade Stuff
Pinball Arcade - great re-creations of classic tables
Pinball HD - excellent graphics and physics
Midway Arcade
Sonic CD
Penny Parlor - interesting
Action Games
Samurai 2
Infinity Blade 1,2 (best graphics)
Other Great Titles to look into
DrawRace 2
Crimson Steam Pirates
Scribblenauts
Stair Dismount
You Don't Know Jack HD
Enjoy