Xigbar.. by far..
Xaldin was pretty hard and he did kill me two or three times my first time through, but Xigbar killed me at least eight times. Though, to tell you the truth, that was namely because of my own stupidity. *facepalms for self* Note to self: Don't drive against.. Xigbar!!!!
Of all the bosses in KH2, he is the worst boss to go Anti-form against, imo. Anti-form against Saix is completely livable and can even be beneficiary. (namely because of it's speed)
Anyways, I tried to drive a few times (when low on HP) and got pwned in no time, once the inevitable Anti-form occured. Finally, I was able to beat him a several tries. Even on my recent playthrough, he's still the hardest. It took me three times, this recent time and two for Xaldin. The rest of the Org. took me only one try, this time around.