Uncharted 2 is the absolute best game on the system, and few would disagree with that. I've never encountered ANYBODY who doesn't love that game.
Beyond that, it's down to genre preference. My second favourite is GoW3, but if you don't like hack and slash, that won't appeal to you. Third is BioShock (the original, haven't played the sequel). And fourth is MGS4, but if you were going to get any semblance of enjoyment out of that, you'd buy it without asking.
Stick with LBP; it's fantastic. Then, I would recommend Uncharted 2, over Valkyria Chronicles (STOP BOOING! It has multiplayer, for a start, and trophies, and also 100 GoTY awards). If you want an online shooter, MW2 seems to be the jeu du jour, but many do prefer BC2. Depends on whether you want small and tactical or immense and tactical.
(Former) Lead Moderator and (Eternal) VGC Detective 
