They're all such great games, I would narrow it down personally to my 3 recommendations: MGS4, Valkyria Chronicles, and LBP. From what I've heard from a friend, the last one really seems to be the game to get for the PS3.

But personally I would recommend to you, 100%, no contest, Metal Gear Solid 4. I mean, damn. If you got a PS3, there's no way you should miss it. I'm not sure how many MGS games you've played, but I wouldn't consider it a probelm, since the first MGS heavily referenced previous games, its story was deeply built around them (not nearly to the extent of MGS4), and most people who thought that was a masterpiece probably didn't play them. I know I didn't.

This post is becoming longer than I thought it would be, and I could go on and on for pages about why you should get that game, I'll just say it's a craaazy portrayal of life, insane story, scenario, characters, with must-see graphical design and completely out of this world gameplay. Get it.