I say Saix, if you're going by actual physical strength. But the toughest for me was probably Xemnas, but he had to be fought multiple times before he went down.
Agh, each of the member's powers and fighting styles were all different, it's hard to compare them.
I don't think a movie would work... but an animated series might.
Actually, if it was something like the Advent Children movie to the FF7 games (taking place in-between games or something similar), then it could work...
Hello! I'm NYUOtaku, but I usually go by my pseudonym, Nana, here on the interwebz. As my name suggests, I am an otaku (hardcore or not is left up to you) of many things, most of which include some sci-fi TV shows (like Doctor Who, Stargate, and Psych), books (Diane Duane and Lisa Mantchev are two of my favorite writers), and animation in general (The Melancholy of Haruhi Suzumiya is my all-time favorite anime, but I also love quite a few American cartoons). But I also have a serious obsession with all things Kingdom Hearts. I'm an amateur artist, video editor, and writer (of original fiction, mostly), and the KH series is what fuels a lot of my creative inspiration.
I'm not really a forum person, and I hardly know anything of such things (I mostly hang out on DeviantArt, YouTube or Tumblr, where forums are non-existent or not the primary function of the site), but I figured I'd take a chance with the KH crowd. What can go wrong when we all like such a great series?
It's nice to meet everyone, and I hope we get along! ^^