Which bosses did you find the most difficult/frustrating in the KH franchise?

What story bosses did you find to be the most difficult/frustrating to beat throughout the Kingdom hearts series the first time you played through the games and why? For me It would be fighting Roxas in 2.5 final mix as I found his attacks quite hard to dodge during my first play through of the game. As for frustrating I would say Vexen, in Re chain of memories as when I first played the game I found it quite difficult to time my attacks so I could actually damage him while trying to avoid being frozen (although second time fighting was much easier , even though it was still frustrating) .

 

Mysterious figure was pretty ridiculously difficult, especially as you had to beat him three times :S I remember on my very first kh1 playthrough I got stuck on Ursula for so so long. And during this playthrough of 2FM, the Luxord Data battle was insane with the timing for his desperation move :S I actually had to get my brother to help me XD I was doing the pause - unpause method to try and get all the Os, and for the last one had my brother unpause it for me while I furiously mashed X. It worked though aha :P

 

for kh 2 it goes by sepiroth.

for KH BBS i would say between young xehanort from mirage for three character
mysterious figure for terra and ventus
and vanitas sentiment for terra.

i manage to beat mysterious figure with aqua because his barrier save me from alot of trouble.
but,it impossible beat him with ventus and terra if he use cloning.

 

Minor spoiler for Kingdom Hearts I

Spoiler

Leon.

For whatever reason I could never beat him.

Secret spoiler for Kingdom Hearts II

Spoiler

Sephiroth.

I don't think I've ever combined so many swear words or said so many insulting sentences as I did with this battle. I managed to beat him after three days and countless tries, but I will never do it again.

Chain of Memories, it was Larxene. I never beat the game because of her.
Can't speak for Birth by Sleep, 358/2 or DDD, I never bought the former and never finished the latter.

 

Mysterious Figure was hands down the most difficult, especially with Terra. His insta-kill grappling was annoying as hell, so was his habit of turning invisible.

There was also that boss in KH1, the one we fight while flying. For some reason managing to hit it physically when prompted required a lot of luck. Chances were you' d swing and miss, which annoyed me to no end.

None of the others were particularly hard or annoying to me once I figured out what to do (though I have yet to play DDD).