Hard to say.. Think it'll have to be a draw between the mask (LoZ:MM), damned 3 formed thing was so hard to beat without the "cheat" mask xD (it was to OP to not be a cheat if you ask me =P) and Dark Samus in Metroid Prime 3.. dammit, after you beat it it possess a giant machine that annoys the shit out of me =.=" (the worst part is that they reuse all the bosses from the previous games.. ok, not literally, but if you compare the bosses, there will always be one with the same attack pattern and weakness in another of the games. Metroid Prime (first form) and Emperor Ing anyone?)