The toughest enemy in Doom. Was described by id as: "A missile-launching skyscraper with goat legs. 'Nuff said". The CyberDemon was the 'boss' at the end of Episode 2 (The Shores of Hell) of the original Doom, and also featured heavily in all subsequent Doom games.

A supremely cool (if slightly small at 3.5") pewter statue of the CyberDemon is availabe in the id Anthology collector's pack.

Even if I agree with the fact the Cyberdemon is a tough enemy in Doom, I remarked that when setting up a level in which he would face the SpiderDemon (so that they'd shoot at each other until one of them would die), the SpiderDemon usually won so I'd rather say that the Cyberdemon is the second-toughest enemy in Doom (after the SpiderDemon).
The fact one could easily get rid of him because of the availability of a BFG9000 in the third episode (Inferno) would just make him look like less dangerous.