I personally found the Heartless in the Carribean World difficult...The Fact that you had to hit it with Magic only until the Coins fell out of it and back into the chest...But hardest Organiztion Member to me was Xigbar...mainly cuz it took so long to do o.o
I beat Sephy at lvl 59 >.>;; (On Proud mode after I learned how to fight him on Standard) But it took me a crap load of tries. Once you figure out how he moves and can counter his reaction command you have him beat. After he powers up tho...Then it gets tricky since he hardly leaves you any time to get a combo in, but still yet same rule applies. Figure out how he moves and you can beat him quite easily, but always bring Elixers as a back-up Sin's Harvest was always his downfall since it takes a lot longer to cast then in KH1