I dunno if it's easy to just pick one, there are tons of characters that are hard/impossible to cosplay. I mean cosplay well that is. You could probably cosplay just about anything, but that doesn't mean it wouldn't suck.
I think the question should be narrowed to "What humanoid character without weird appendages has the hardest outfit to cosplay". In other words, just in terms of hair/clothes, what is one of the hardest to cosplay. Cus otherwise you could throw out weird stuff like various demons and robots and aliens and such. Imagine trying to do an accurate cosplay of the monster forms from like, Project Arms (not that anyone would recognize it because I swear almost no one has seen it) or Heroic Age or the like?