It's hard to decide!!
Mana's style is definitely amazing, and not only the M10M one, also the costumes and make-up in GLB. And not to forget the Malice Mizer stage! I think he's driven many fans into cosplay.
As a whole band, I'd pick Versailles. They make a stylish quintet and even exaggeration (such as Jasmine You's head "stuff") looks good on them. Hizaki and Kamijo are really interesting individually too.
And of course I have to say Kaya. Every time he changes costumes during a live, he looks splendid whether he's drag-queenish, princess-ish, school-girl, or a sexy boy! I don't know anyone else who can cross gender boundaries that elegantly without losing "Kaya-ness" in the process. He's the ultimate "drama queen".