The Cyberdemon
One thing comes into mind when he comes stomping into view. "Aw crap," being number two with "Oh shit," being number one.
The Cyberdemon is indeed to be respected...if not feared due to the cannon for a right arm and hand. One rocket from this guy will definitely make you feel worse about life.
If the classic Cyberdemon is bad then the next-generation Cyberdemon from Doom 3 is much, much worse.
The Cyberdemon in Doom 3 is as tall as a three-story building and conventional weapons doesn't do squat against it. Only thing that can does work is the Soul Cube.
Either way, the Cyberdemon (aside from the Spider Mastermind), is something to be feared...and respected.