I have gone over this in my mind many times by now just for the fun of it, but there are many different ways to rank them (from importance in plot, to weapons, attitude, etc.) Overall just for fun I would probably go with:
Top Zexion - beats out 2nd place because his attitude is too himself and he has the coolest weapon by far Larxene - best attitude because she doesn't care about anyone but herself and she is quick, but her weapons pale in comparison to Zexions'
Mid-Top Marluxia - still very cool, nickname helps "Graceful Assassin" with an awesome weapon and cool hair, but the name pronunciation angers me Xion - 358/2 Day ending made me cry cause I never realized just how cool Xion was Xigbar - Far-range fighter (like me) and he has a cool past that played a fairly large part in TVA's story
Middle Axel - played a big role, and had some cool lines, and I like his weapon, but thats about it Saix - purely because of his drive and boss battle in KH 358/2 Days did I find him cool. I also enjoyed his attitude in Bbs Roxas - Sora's counterpart, so he obviously has a huge part in storyline, but overall I was fairly dissapointed with how his attitude towards things was so predictable and I found it kind of boring, but he is still cool Luxord - just makes Middle becomes I love his use of cards, but too little is known about him and he is a gambling addict
Mid-Bottom Demyx - plays little role in game at all, but I love water, and I love his aloof, uncaring outlook on everybody and everything Vexen - brains behind team which is cool, and his ice shield isn't bad, but I find his snippy personality annoying sometimes Xemnas - loses so many points because he isn't a very cool boss nor leader, his weapons are just OK, but I never saw him as anything too special among the Org.
Bottom Xaldin - even though I know his background story, he is still boring and his weapons moderately interest me, but not enough to gain him too much likeableness Lexaeus - Aeleus is kind of boring to be honest, and his weapon is more or less plain and boring, his boss battle in Re:CoM and CoM was laughable and he isn't that cool in my opinion.
Vexen, for sure. He's got that....smugness and smartness to him that makes him bad-ass....That, and the fact that he uses a shield. He's like an evil Captain America.....without the patriotism and all that bullcrap.