I have a looot of cosplays I'd like to do in the future, but I think the hardest for me are going to be
Alice from Record of Agarest War or
Yukimura from Sengoku Basara. Alice has a lot to her outfit (accessories, clothing pieces, small details, a weapon, etc.), while Yukimura would require some open chest binding/make-up magic to make me look like a somewhat well toned man, ha ha. Yukimura has quite a few armor pieces as well.
They are ambitious for sure, at least for my current skill level.