Simple question, which is your favorite entry in the series and why?
Birth by Sleep - Easily the best gameplay in the series with one of the best stories as well. The set-up of the game is nice, the cast is cool, the story is pretty dark and emotional and the gameplay just rocks from beginning to end. Finally reveals several looming questions in the franchise and also set-ups what KHIII is going to be all about. The tragic story of all three main characters is some of the most emotional stuff in the series. The new Drive system is also amazing, with a much smoother interface.
358/2 - A very emotional story centering on fan favorite Organization XIII, and pretty much proves how the concept of the Nobodies is one of the more thought provoking elements in the rather cutsey franchise. The gameplay is pretty solid too, though the camera is an issue and god knows the DS can't handle the ambitious design like SE probably wanted it too. Still, the customization system is actually pretty ingenious and the new way of approaching magic was very fresh and original. The game has easily the most gut-wrenching ending in the franchise and having the knowledge of Roxas' fate in KHII only makes the ending more tragic. I honestly thought I was going to hate Xion but by games end, she has become one of my top fave characters in the franchise.
Chain of Memories - The first game that convinced me that maybe KH won't just be a silly novelty series. The "card" mechanic is actually well done and finds a happy medium between strategy and keeping with the series action-rpg roots. The introduction of Organization XIII was a nice boon since Ansem was a nice villain but kind of showed up late in KH1, it was nice to have a set of consistent villains that actually felt a bit menacing. Being able to play as Riku was a boon as well and this is still the only game where I felt the summons worked... The story was also pretty twisted, not what I expected and the games endings, while not as tragic as the first game, still left you clamoring for more which is how you should do a series.