Possessed riku was the hardest for me, it took me (no joke) like one and a half years to beat him. Sure six months of that was refusing to play again but then one summer I got myself to level 100. Then went to battle to him again and I finally won.
Oh, yeah, I also remember having a hard time with Rikunort (the first and second one at Hollow Bastion). At took me, at least, three tries per fight. *shrugs*