i'd have to say the true endboss of demon's crest. we're talking HARD. he had 2 forms, and you could only hurt him in one. and every time he switched to that form, you had to be quick and hit him, because he was making the walls, ceiling, and floor turn into spikes.
he was so hard you could only fight him if you had found EVERYTHING.
god that game rocked. great music too, deserves remixing.