LittleBigPlanet
First the level creator is very detailed you could change nearly everthing inculding enemies. With the online option you can see you friends, friends best and most liked levels. The main story mode is a game in itself. If you just did every level once you could finish it in a day but to get the acheivements you will have to do the levels multiple times. When you play online you may also see some of your creations on other peoples levels which is great.
IGN’s review