I would recommend you purchase Ratchet and Clank Future: Tools of Destruction. You might try renting Uncharted and Heavenly Sword to see if they are your type of game.
If you like FPS, Warhawk is awesomeespecially since Insomniac continues to release expansion packs.
Also, I'm a big fan of social gaming. So SingStar and Buzz! Quiz TV are must-haves for parties.
As for the PSN, the only title I would really recommend is PixelJunk Monsters. If you have disposable income, you may want to try Flow, Super Rub-a-Dub, Echochrome, Elefunk, PixelJunk Eden and Racers. However, in my opinion those specific titles get old quickly.