Mine would have to be the enforcer. I personally do not think it is much of a threat in the sequels, since the projectiles can be dodged easily with skill. But the M1 enforcers fire frequently and their projectiles are almost instant-hit, plus the rather wide spread makes it quite difficult to dodge. That added to the fact that enforcers sometimes come in groups. Call it an unexplainable and irrational fear, but I always get heated when I see one (sometimes even when they're dead). Doesn't stop me from applying my effort towards fragging them, however.
In my book, juggernauts are spazeroids gone supersize. Slow, stupid, and predictable. You can simply run behind a corner, jump out, pepper him with anything you can quickly (preferably missiles and/or grenades), duck back behind the corner, repeat until he crashes down.
I may be a pansy in the sense that I barely use my fists unless I must, but I do great with weapons in Total Carnage.
Of course, in the end the enemies from Marathon on TC are far less dangerous than UT's godlike bots, but even THEY suck.
-Dael the Demon. "I love to kick vidgame enemy butt."