CRAZY DUMB ZEALOT
Former Human: just an introductory joke of a monster. Should mostly appear in early techbase maps as a small threat, in reasonable numbers. They might be effective when placed in alcoves and such, against an unarmoured player. They really have no place in medium-hard maps, they look like an anomaly there.
Former Human Sergeant: Much more versatile, they can fit in any place. They're somehow like the Wolf3D enemies, just without the teamwork factor, which means they shouldn't be put in hordes.
SS: I've really rarely seen them, probably because of Nazi sensitivities. But when they appear, they tend to appear in hordes in abstract-looking Doom 2 maps in specially crafted places. You can't tell if they're firing at you or themselves, which has a psychological effect. Otherwise they're like (much) weaker chaingunners, good at being a mild priority.
Imp: The most common monster in Doom. Sometimes they're pushovers though, especially when placed all in front of you.
Heavy Weapon Dude: In hard maps, they almost completely replace the sergeants, and for good reason. They're a popular choice of a monster behind wall curtains.
Lost Soul: Probably the silliest monster in Doom. Good ammo sponge and interesting flying dynamic if left time. Good in alcoves, especially 16-wide ones.
Demon: The typical hell dweller. Great as a support challenge on you, and usable almost everywhere, though still generally a low-end monster.
Spectre: They're at their best in slime or ashwalls. Hardly visible there.
Revenant: The grim reapers of Doom, many time my session ended as I was carelessly blown up by their rockets. They're also very noisy, and look very ugly in the Doom environment, which has a psychological effect too: can make me dislike Doom 2 for its ugliness. They're versatile which means they can appear in many places, but can make the map feel painful at times.
Cacodemon: The typical flying monster, very maneuverable, grinning at you like that. They're very painful at melee range.
Pain Elemental: Weak as hell at melee range, they're at their best when you can't reach them, but can't easily focus on them either (because of other more immediate threats). Generally the gameplay against them is cover-shooting.
Hell Knight: Can be used as a replacement for the imp in advanced maps. Otherwise they're great in cramped places. Their scratch is very powerful, and hard to avoid if you try to run past them.
Arachnotron: Not very maneuverable because of their large size, and not very resistant, but their plasma attack is useful. I admit I've rarely seen them used effectively: most of the time they're easily dealt of by most of your weapons. If they do manage to take a shot, they can keep you cornered.
Mancubus: Useful in large areas where their spreadfire can keep you retreated. Can easily get along with other monsters if they're placed apart, without becoming weaker because of distance. They're boring if the player only has the shotgun, though.
Archvile: Pretty much a boss-level monster, there should always be some cover against them (failing that, the player needs a plasma gun or BFG and fast reaction). Their resurrecting power can be their weakness, as it keeps them busy.
Baron of Hell: Good as a replacement for Hell Knight hordes in late maps, can mean "we really aren't going to joke here". Also good as support monsters if you want to keep the player busy.
Spider Mastermind: Generally considered the clumsiest. Best if appearing as a boss you have to fight directly. It often needs to be aided by environmental effects (slime, Boom wind). Never place it on the same ground as imps, demons, spectres, knights, revenants, barons. They easily get jammed by the melee monsters, and it's never cool when a lone spectre solves the whole problem for you. They can live with mancubi placed on higher ground, because the mancubi will focus their fire on you, while you're probably away from the spider, while the spider will not be able to accidentally hit the mancubi. Kind of useless with arachnotrons, even though unfortunately they're often featured that way in maps. Also not very useful as turrets, unless you can't reach them easily (even them, they might help you more than harm you). Useless if the player has any plasma gun and ammunition.
Cyberdemon: Much more versatile than the spider, a threat at any time. Of course, they're best when they're free to roam, but can also be used as turrets.
Boss Brain: Very much the ultimate evil, and the very thing you're moving against in Doom 2. If there's one around, it means the map has a short time limit during which you need to figure out the shit to do. And that's usually not easy to accomplish, requiring good reaction inside an increasingly infested level. And there's lots of variety on how to use this monster: can be simple like in Icon of Sin, can be multiplied, can appear in many places that can trigger if you do something wrong etc. Of course it can only be reliably used in MAP30, due to the missing telefrag otherwise (though there are cleaning solutions, such as crushers in remote teleport destinations…)
Automatic Wolfenstein - Version 1.0.1 - also on Android (Updated 28/03/2015)