Phantom took me forever to figure out (considering how I was an RPG newb, during my first playthrough of KH2), but I think I liked Kurt Ziza the most, out of all the KH1 bosses. Sure, I was able to win the first try, but the battle has always had a special place in my heart, especailly the intro cutscene, which has a knack of making me jump, even when I fought him for the fourth time.
In other news, Dragon Maleficent was easily my favorite Disney boss and I think I have to give third place to her. That battle was truly epic. Too bad we never got to fight her in KH2, but then again, do we really want to wear the welcome out that much? Sephiroth gets second, since KH1 Sephiroth was awesome and the battle was better than KH2's. Fourth goes to the second Riku battle. It may of not been all too hard, but the battle just had that epic feel, just like with Dragon Maleficent and Kurt Ziza. Don't get me wrong, the battle with Sephiroth was pretty epic to, but I liked more for the challenge. Finally, in fifth place was Phantom. The battle was and still is pretty tricky, but that's what makes it so good. Besides, it's a battle where you actually HAVE to use your magic.. or you'll get owned, real fast.