I would say buy:
Uncharted 1&2--some of the most fun I've had playing games this gen[and I've only played U2's multiplayer, lol]
LBP--very fun if you can get over some goofy["realistic"] platforming controls and have fun friends to play with
MGS4--you already plan to buy this so no explanation needed
Ratchet and Clank--I've never played any of these games, but the footage for the newest one makes me all giddy inside
And that's all I've got. I haven't played too many PS3 exclusives[I'm a 16 year old, working a weekend job.. not too much money to spare], so others probably have a better list.