If I had to rank them:
1. Kingdom Hearts
2. Dream Drop Distance
3. Chain of Memories
4. Re:Coded
5. Kingdom Hearts II
6. Birth by Sleep
7. 358/2 Days
To be honest, I enjoy Days' story more than that of BbS, but the structure of receiving a little bit of story + new mission repeatedly isn't conducive to an interesting game. I do like what they do with the Disney worlds in BbS, but that's about it. You're saddled with 3 characters who I have lukewarm interest in, 3 individual stories which--while they meant well (like the 2nd time visits in KH2) to give us more game to play--makes the game last even longer than I would've wanted. Not to mention the overall coldness and emptiness that pervades the game for me. It's mostly because there are no party members, but probably more so because the characters themselves don't have personalities that fill the void like Sora, Riku, or even Roxas do.
Kingdom Hearts II is a fun game if you cut out the Disney worlds entirely. And since part of the reason I play these games is to see interesting things done with the Disney worlds/characters, of course that doesn't lend it to me anyway. Throw in all the other issues, and that's why it's so low for me.
Re:Coded and CoM have the re-hash going on (the former is worse than the latter in that regard, imo), but I feel like they each provide strengths to balance that out--I especially think the writing of the re-hashed Disney levels fun, which makes the repeat less difficult to deal with. CoM utilizes the Organization better than the other games they're featured in, and the interaction between Sora, Donald, Goofy, and Jiminy (and Mickey and Riku on the flip side) makes that game one of my favorites.
Kingdom Hearts and Dream Drop Distance are the two games that encapsulate best what I look for in this series--interesting Disney levels and Sora-Riku interaction. Of course, I think they do more with the Disney (and FF) characters in the original game than the latter, so that's why I rank it higher. Plus, DDD is bogged down by the Xehanort melodrama at the end.