Project Brutality for Doom. It's like Brutal Doom on steroids punching Hitler through the stomach like it's Kung Pow!: Enter the Fist.
It's a standalone mod (doesn't require Brutal doom to work) and is built off of Brutal doom v20.
It's literally the most extreme, over-the-top, laughably and satisfyling violent thing I've ever played in my life. There are variants of every monster in the game and multiple ways of spawning (I use Default Random for a somewhat different experince each time I start a wad). There are waaaay more weapons, all of which have alt-fire/weapon special abilites. More melee combos. Mach speed sliding kicks (crouch and do a kick; this makes level two of Plutonia so fucking fun).
Remember imps? They can climb across the ceiling and shoot fireballs, as well as jump down the floor/jump back on the ceiling. Try playing the level in Doom 2 where you can crush an arachnotron by fliping a switch - I was seeing imps jump up to that level's super-tall ceiling llike it was DBZ meets the Excorcist.
Oh yeah, and just about eveything in this mod is customozable from the "Project Brutality" menu during play - you get to decide if Imps can jump on the ceiling, if certain variants spawn, and more.
And with all that said, I still don't think I've hyped this up enough to properly express how absurdly fun it is. PLAY IT. PLAY IT NAO!!!