Personally I would highly recommend:
- God of War III
- Heavy Rain (Highly recommended by me but might not be your cup of tea)
- Uncharted I and II (Beautiful visuals combine with a great story. Pretty easy to beat on the lower difficulty levels. It's considered by many to be the most visually advanced console game on the market, well the second one, first one is still pretty beautiful in terms of graphics.)
- Metal Gear Solid 4
- Dead Space (probably Dead Space 2 as well but I can't speak from experience)
- Mass Effect 2
- Infamous (GTA style with super powers, lots of fun. Lets you decide weather or not you want to be a hero or a villain)
- Fifa
- NHL
- Assassins Creed II and Brotherhood
- Bioshock
- Mafia 2
- Dead Rising 2
- MLB 10/11 The Show
- GTA IV
Most of the acton/adventure games I've listed you could easily beat with a one or two week rental, so Gamefly might be a good option if you're penny pinching. Also I've never played it but Little Big Planet comes highly recommended. Mass Effect 2 you would probably need to own to beat but I've loved it so far.
Here's some other games I've tried that you may end up loving but were either mediocre or average when I got around to playing them:
- Killzone 2 (3 is coming up as well, I would only rent this for the sake of seeing how far the PS3 can push an HDTV but other then that it's overrated and tedious, I traded it in after a week)
- Final Fantasy XIII
- Fallout 3
- Resident Evil 5
- NBA 2K11