Duke Nukem 3D - Mental Heavy (PS1 version 3)

The game features a wide range of monsters, some of which are aliens, other mutated humans (the LAPD has been turned into "Pig-cops", a play on the derogatory term "pig" for police officers, with LARD emblazoned on their uniforms). As is usual for a first-person shooter, Nukem encounters a large number of lesser foes, and a small number of boss enemies (usually at the end of chapters). Like Duke, these enemies have access to a wide range of weapons and equipment (some weaker enemies have jet packs).

[@bunnehboi1985]

Nothing like blowing up a nuclear reactor on the moon to this epic music. But there's two things that ruin Lunar Reactor for me...
1: PROTOZOID SLIMERS
2: *THREE FUCKING BATTLELORDS!!!*
But this music makes up for both of those!

[@0neDoomedSpaceMarine]

One time, I actually had a Protozoid Slimer come up behind a Battlelord, crawling along the ceiling, then dropping down.
The Slimer fucking *_ate_* him whole, in just a second.
I have never had that happen ever before, and not since, but it was bar none the funniest goddamn thing I've ever had happen in Douk3D, perfect anticlimax to what was gonna be a tough fight.