I personally liked 358/2 days. And I agree, the spin-offs fill in some of the story, but they can get old after 2 or 3 of them. And apperently some of you don't realize that Re:coded acually takes place AFTER KH2 and it does indeed advance the story, and tells you what was written in King Mickey's letter, (the one at the end of KH2). As for KH 3D I'm not sure, it was only recently announced, and I know Sora & Riku had their KH1 outfits on but they did in Coded too. So I suppose we'll just have to wait and see.
EDIT: This might help:
http://kh13.com/mybb/showthread.php?tid=7101