Hey, I was beginning if none of the games I voted for made it in! I love Birth By Sleep. It definitely is the turning point of the series into the "wait, what?" kind of story, but I still really appreciated how much of a breath of fresh air it was to introduce new protagonists and establish more of the Kingdom Hearts world before Sora comes into the fold. I think Terra, Ventus, and Aqua are all great characters in their own right, getting Xehanort more fleshed out was pretty cool (especially with Leonard Nimoy's performance), and playing through the secret ending of KH2 was amazing. I can't imagine how many times I watched that FMV on YouTube back in the day.
Even though it's not a numbered game, I barely count BBS as a spinoff, really the only differentiator is that it uses a different combat system than 1, 2, and 3 (and it being on PSP, but whatever). I'll always prefer the combat of 2 and 3, but I still really liked the command system. It gives you access to some cool, unique moves that don't show up anywhere else, and even though you probably need a guide to figure out how to create some of them, I don't hold that against the game too much.
When I was going back to get the platinum trophy in each KH game, this was definitely the one that I realized had my least favorite side content. I couldn't really get behind the monopoly thing, the minigames were all kinda so-so, and I thought the Mirage Arena was fine, but didn't have the same charm as the Olympus Coliseum tournaments. But overall, I still respect the hell out of the things this game tried, how it built out the KH universe, and I'm also just a sucker for how bittersweet the story is in this one.