Mana gets my vote, though I have to admit, its mostly by association -- the whole tanned gun toting 'Toko thing, as you might have guessed. She was barely in the first Negima anime, and I'm not far enough into the manga to have seen any development for her yet. I'd like to get back into the manga, and I'm not picky about reading everything in order, so if anyone can suggest a good starting point for a would-be Mana fan, 'twould be most appreciated.
In the meantime, Mana wins on the basis of sheer sex appeal

I guess one could say its a matter of aesthetic. There's just something about girls with long, dark hair, and either really pale or really tan skin that just does it for me

There is a bit more to it, of course, even with the little I know -- I like her mysterious personality, and the element of danger. I can't stand the whole fake "tough girl" stereotype, but Mana is the genuine article. I imagine her and Black Lagoon's Revy sitting down together to talk shop about guns, killing, maiming, etc.