Someone mentioned the Archvile earlier, and that reminds me of the Mother Demon at the end of Doom 64, who's basically the Archvile on steroids. Not only did she resurrect every dead demon from the Mars Phobos and Deimos bases but her fighting style combines the flames of the Archvile with the homing rockets of Mancubus or the Revenant.
I figured Doom 64 could count as being part of the Classic Doom era for many, since it was released in the nineties, used an engine close to the original Doom and was a continuation of the storyline of Doom and Doom II.
