Well, here's my choices..
The Gathering Place - because if it's the main world, it should have a decent story.
Twilight Town - I would love to find out, what purpose this will have in BBS.
Radiant Garden - Possibly, we can meet a few of Organization XIII's members (and Ansem the Wise).. before they became nobodies. Who knows, maybe, there's a few of them, besides the original Apprintices, there..
Sleepy Beauty - If only for Maleficent's purpose and her dialogue with Terra. (I want to know) Also, if it appears, the Dragon.. of course.
Destiny Islands - This would be great because we can finally know, just who, made that keyhole appear in Riku's view.. in that flashback in KH1. Also, Ven's dialogue, there, is something that I want to know.