Umm some people may laugh but I don't care... The final boss on Pandamonium used to scare the BEEP out of me. I'm aracknophobic and the idea of a giant robotic spidery looking thing with one huge eye and a really annoying laugh did not go down to well when i was younger.

But from newer games i'd have to say Pyramid head from SH5, I think it was they way he moved, dragging that big, old and bloody sword around behind his back. *shivers*