good question, I already have littlebigplanet, it's a great game, the best out of all of them. Infamous is an action game were you play the role of an electric superhero type guy. It got good reviews and has some rpg elements such as skill development. Dead nation is a zombie survival arcade game. It is is a smaller game, and got decent reviews. It's top down and not as in-depth as infamous. Wipeout recieved great reviews as well and is a futuristic, intense, racing game. Ratchet and clank is a fun adventure game, comparable to the old crash bandicoot and sly cooper games.
These are all pretty good titles, very generous of sony. Infamous, LBP, and wipeout got dazzling reveiws, while quest for booty and dead nation got solid reviews. If I didn't own any of these games I would without a doubt choose Infamous and Littlebigplanet. I already have LBP though, so I think I'm gonna get wipeout.