I haven't played Doom in a REALLY long time, but the Cyberdemon always stuck out to me and now that I think about it, I never did beat it. . . always liked Dark Forces more.
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My favorite doom story is when I was little my friends and I,found a disc with doom 2 on it and although my parents frowned on it we would play it every now and then. We would do a strange two player combo of one mover, one shooter (lots of disagreements on what needed to be done happened I assure you). One day we were exploring around an area we thought was clear when we suddenly got shot in the back. Whoever was moving did a quick 180 and I shot it with what we had equipped at the time, the BFG 9000. Turned out to be a standard mook and we sat there and watched him melt from the overkill, then laughed about how outrageous it was to kill the base level enemy with the most powerful gun in the game. Good times, good times.
It's worth beating (some fun levels and weird architecture in the last episode of the game,) but the Cyberdemon is easily the highlight; the Spider Mastermind isn't nearly as cool or as fun to fight. Which I guess is why they dropped it and just used the Cyberdemon as the final boss when they remade it as Doom 3...