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Metal Gear Solid 4 is a must. Also pick up Burnout Paradise if you don't have it (I logged in over 12 hours on it already and still have a ton of stuff left to do). Little Big Planet is worth it, but probably not at the full retail price if you aren't going to be creating levels. It's a great party game.
LBP is only worth it if you like sitting down and wasting time. If you're into shooting up terrorists and raping prostitutes for the lulz, get GTA4 and CoD4. MW2 looks more than promising for a shooter.
If you want exclusives, go for MGS4, Killzone 2 and Uncharted. If you're just into Next gen, then Fallout 3 and Dead Space are my favourites so far this gen.
LBP was a bit of a let down for me, I got about 2 days of enjoyment out of it. I've been meaning to go back to it and try some of the levels that were created while I was away but I just don't have any draw to play it again. I would recommend renting or borrowing it first.