The Organization XIII member battles are incredibly fun in Kingdom Hearts 2. I believe you fight them in other games, but this is just for Kingdom Hearts 2. Anyhow, you fight 7 (I think) members of Org. 13 in the game. So just vote who you loved to battle the most! ...And if everyone votes for Final Xemnas, try to be different...

Axel - Fighting Axel was just... a really fun battle. Got it memorized?
Demyx - Those water challenges were so fun! The second battle was really difficult, but I still loved it!
Xaldin - Those spears, man. Those spears... So fun to use the Jump ability so much!
Xigbar - This guy was damn tough to beat, but he was equally as entertaining to fight.
Luxord - Luxord's battle was pretty fun. I loved the unique gameplay style used!
Saix - Though the battle was fairly short, it was still fun to fight him with him going into Berserk Mode a couple times.

Skyscraper Xemnas - This took awhile, but it was pretty cool fighting Xemnas for the first time.
Throne Xemnas - The first battle was small and lacked content, though it was still quite fun, but I liked how the second battle required you to jump off of skyscrapers while flying around.
Nobody Dragon - A very unique battle style, but still pretty entertaining to fight. That Megalaser...
Final Xemnas - This was the ultimate battle between light and dark. It was very long, very fun, and the two "intermission" attacks (when you play as Riku and when he uses the Ethereal Blade [Laser] Storm) were also an awesome experience.

So how about me? Although the final battle with Xemnas was totally awesome, I'm going to have to say that the battle with Xigbar was probably the funnest. It was extremely difficult (or maybe I had a horrible strategy?) and extremely long, but it was still very fun for me to do. But I kept going into friggin' Anti-Form, which is the most annoying thing in the world unless he's using his special ability that shoots a huge amount of lasers at you.

Demyx was pretty hard when I fight 2nd time against him, it was good challenge had to fight couple of times till I won him.

But I have to say my favorite fight was against Xaldin he was the hardest to beat when I played the game first time, won him because King Mickey came to help me after I went K.O.. I was like wtf your doing here..

I liked to battle against all of them but those two were the hardest=favorite, because I like when the game is challenging..

Ugh Xigbar was by far the worst battle. I think my favourite battle would have to be either Final Xemnas, Axel or Saix. Xemnas was fun because of the setting battle style and the different reactions you had to face. I also liked playing as Riku for a bit. Axel was fun because you got to play as Roxas with the 2 keyblades and he was just a fun boss overall. Saix was also fun to just go berserk with too. I had to use the Mickey save to beat Xaldin though. But Xigbar....freaking Xigbar.

Blubcreator : I know, I hate Xigbar. He's probably my least favorite battle. But he's also my favorite because he's so dang difficult and fun

Aside from Luxord's battle which I enjoyed because it was very unique from the rest and took a while to figure out, I hardly enjoyed any fight against Organization XIII. Xigbar's fight was so-so, since it was nice and challenging, but once you got the hang of it, it started to become boring. Xaldin and Demyx's fights were borderline cheap, with Xaldin's undodgeable dragon-laser-thingy, and Demyx's 10 water clones in 10 seconds... Axel's fight was pretty cool though, I must admit.

Still, I pick Luxord's fight. It was a fun little change, and got me by surprise.

Fighting Axel when using the two Keyblades as Roxas was awesome and really fun, I love that battle, made another profile on the game, got to that part, saved, now I can do the battle whenever I want .