Definitely get Heavy Rain, it's one of the best PS3 exclusives, the tone and feeling of it's story and the characters are really great, despite the slow start. Also pick up Uncharted 2, that game is awesome. 3D Dot Game Heroes is a lot of fun, and you can't go wrong with Little Big Planet.
However, if you haven't played the previous games in the series, DO NOT get Metal Gear Solid 4. You will not understand a bloody thing in that game, and the game itself is kinda sucky.
However, if you haven't played the previous games in the series, DO NOT get Metal Gear Solid 4. You will not understand a bloody thing in that game, and the game itself is kinda sucky.