Well I haven't played BBS, but from what I've heard of it it looks amazing. Here's my list so far:
1. KH1: Nothing beats the original! It had puzzles and platforming elements in it, which gave it a bit of a challenge. Not so much a challenge that it was really hard, but a fun challenge. Plus the Disney characters played a much more important role than in the other games. The storyline was pretty good too. Overall, this was a really good game.
2. BBS: From what I've heard of it, BBS sounds awesome and up to par with KH1 (it might actually change to being my favorite game once I actually play it.) I heard the gameplay is awesome and that it's not just a "Press X and /\ over and over again until the health runs out" game like KH2. In fact, from what I've seen the health bars for the bosses seem rather low and they're actually a bit challenging.
3. COM/ReCOM: I really liked the storyline. I didn't hate the card system either, actually. It was just hard to get used to. But once you did, it was actually a lot of fun (and again with the whole challenge element to this) You actually had to plan what cards to put in what decks and so on and so forth.
4. KH2: I didn't HATE KH2 like a lot of people here do. I actually really liked the storyline of this, plus some things were definitely improved (like Gummi Ship sequences and the fact you can open treasure boxes during battle. Oh, and let's not forget the camera) But the challenging element that was in the first two games kind of disappeared. Instead, bosses with ridiculous amounts of HP were added and it was just a matter of pressing X and /\ over and over again until the health runs out. Also, the need for magic (save for Cure) kind of vanished.
5. Days: I didn't hate Days either. It's actually grown on me a bit. The only thing I couldn't stand about it was the repetetiveness of it. A normal mission day for Roxas would go like this: Get mission from Saix, do mission, learn life lesson about hearts or love or something, then eat ice cream. The game was also a bit choppy. The plot would start out slow, then get fast for a while, then go back to being slow. It was a good game, just not as good as the others.
So yeah, BBS looks to be one of the better games in the series from what I've seen of it.