BIL wrote:
Rifle/SG/Chaingun = useless alone. worse than useless in packs.
Imp = lmao
Demon = an unarmed monster to pick apart at range, oh noes.
Spectre = a poorly camouflaged unarmed monster to pick apart at range, oh noes.
Lost Soul = a slightly more insistent unarmed monster, A for effort.
Caco = a flying Imp, oh noes.
Baron = a damage-sponging Imp, oh noes.
Knight = an Imp, oh noes.
Cyberdemon = its time to circle strafe.
Spider = its time to circle strafe.
DOOM 2 originals = generally excellent! BD gives them a number of nasty upgrades to ensure they still equal or surpass the now not useless D1 set. Not seeing a problem.
Since you edited this in:
Zombieman -- usually there when you need something that's not a threat but actively takes up space. Think of him as a "meat wall", generally.
Sergeant -- high burst damage and threat at close ranges, priority target in such situations. Also, frequently used just as a way of giving the player some ammo.
Chaingunner -- high priority target, area denial. When you have some of these guys, they either have to be the top priority (useful for giving enemies such as PEs or Viles time to work their magic), or they're flat-out area denial, saying "you can't come to this area without getting shredded, stay behind cover".
Imp -- need limited space to create pressure, but in tight spaces with tightly controlled weapons/ammo, they get the job done. Also, good for "gradual attrition" sniping, and also great for keeping that "a body hits the floor every few seconds" pace going.
Pinkie -- runs interference on player movement, and is reasonably beefy. Using a nice, thick horde of them to screw up player movement around Viles or Cybs is a classic.
Spectre -- if you're playing in GL ports, these guys are easier to see than they are on an authentic renderer. They definitely catch people out in classic video settings. Particularly fun to use sneakily in levels that prioritize rocket-launcher action.
Cacodemons -- pinkies that fly and shoot fireballs, but even beefier. They also don't work correctly in GZDoom, because in that port you can just run under them lol. Good for limiting a player's movement and smothering him. Flight means that they can follow a player around well in a map that's designed for them; as such, limiting ammo tightly and unleashing a Cacoswarm is a great way to bully a player around a map, and can also lead to ambushes from unexpected angles.
Baron/HK -- the nobility is threatening in two situations. One is when used in very tight areas; having an advancing Baron or three in a hallway that's tight enough that you can't slip past them is a classic fight for a reason. The other is when you need to conserve ammo; it's tricky to melee more than one of them at once, and the punishment for fucking it up is high (this is a mainstay of the Chord series).
Cyberdemon -- sometimes placed as a tool for the player to use (infighting ahoy!). Threatening in tight spaces where you can't really get away from a wall, because of their splash; also threatening in hallways, where circle-strafing isn't an option (think about that one at the end of the hallway in Plutonia map 32 for a great example). Great for times when the player is likely to be distracted by other threats. Particularly hilarious when teamed with Arch-Viles; the Vile reduces your movement area to nearly nil (gotta stay behind that pillar!), then the Cyb fills that area with a rocket.
The Spider Mastermind
is legitimately really hard to use well, and doesn't fill much of a niche, but you can't circle strafe him in vanilla -- he's a hitscan. Unless you meant "arachnotron" by "spider", in which case, anyone who just dumps them in the middle of a field to get strafed is doing it
entirely wrong.
The higher DPS inherent to Brutal Doom really screws over the Doom 2 squad. Also, the changed Revenant missiles don't really fill the same role; they're not as good as pushing a player out out of cover into the maw of a waiting Vile/Arachnotron/Chaingunner brigade.