I'm going to be picking up a PS3 in the coming weeks myself, so I'm in the same boat as you (topic creator). It'll actually be the first current gen. console I own, so I'm looking forward to seeing my grades and social life crumble because of Oblivion. If you're into any genre and will play anything, Oblivion or Fallout 3 will probably be the longest bang for your buck (I'm on a limited budget game-wise, so that's my mindset going into any game purchase).

LBP is a lot of fun, but I can't be sure if it's just the novelty of it because I've only had limited time with the thing. I guess your mileage will vary depending on how into trying out user content and building your own levels you are.