I honestly have to agree, 100% with parnakas. Little Big Planet will last you much longer than any of the others games coming out. Especially considering it's level designing and sure-to-be-amazing online content.
Although im NOT saying the other games are a bad choice, because I'm sure they are going to be outstanding as well, but if you got a game that will only last a week, then you're stuck feeling bored after completion. However, with LBP, you can create new levels for you to amuse yourself for quite a long while.