http://www.gamespot.com/features/ten...ins/index.html
I thought that was actually a fairly interesting article, and wondered if there was anything in particular to be added on? They're not really looking for like, the most villanous villans, but the exceptional ones.... uniqueness is what I'm thinking of.
Of course, I'd have to say the CyberDemon. He wasn't even really a single character in of himself, just a type of monster, but in any level where he was dropped off, he was pretty clearly the level's 'boss'. That is very impressive reusability. Also, he works just like a good bad guy should.... he's a constant obstacle to the player.
Finally, the cyberdemon seems to have been the first high-difficulty monster (not boss), and the rest of the FPS games maybe can be seen as following the tradition of the Doom series in that respect.
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The One Who Guards the Deeps