When I beat him, I was happy, because I knew I accomplished something difficult. It's true that the 18000 EXP points reward was pale... but the battle accomplished its objective : keeping you interested and excited, as any good video game battle. HOWEVER If Final Mix, I was actually frustrated when I won. Even if you get the bonus scene and the One-Winged-Angel awesome Keyblade. Why? Because they made the fight so much harder, and I could only win by spamming cheap tactics, and not by using skill. It took me 3 tries to beat him in ordinary KH, without any past experience. Took like 60 to beat him in FM, already knowing his moves by heart. But hey, it was worth it. The One-Winged-Angel Keyblade is too cool.
I would say Xemnas, because following the BBS theories, I realise that he was truly determined to achieve his goals... whatever they were. He set up the Organization, brought chaos to the worlds, and manipulated Sora... but if we were to unlock the mysteries of his past and former self, I'm sure we'll find an individual with a heart as strong as even Sora's.
