Disclaimer: The game I'm about to mention is one of my favourites when considered as part of the series, and it's still nowhere near the level of awful as some others in this thread.
With that said, Kingdom Hearts 1.
For the simple reason that, compared to KH2 and BBS, it has not aged well at all.
The story is a bit childish, though it's still better than some modern game stories, and has great, emotional moments. The combat is, as to be expected, not as tight as it should be, and not very deep.
On the other hand, KH2 is still as awesome today as it was 6 years ago.