All three of her forms are stunning; but I'll have to say I'm partial to this one. Not only do I prefer Aesc/Altria Caster to Morgan, but there's also a certain uniqueness to the typical witch hat + floating book + staff; in a universe where almost every magus dresses either like a normal person or like some quirky cosplayer.
I would assume there's a pretty even distribution between the three of them, since they all have their own appeal.