CRAZY DUMB ZEALOT
The wraith is very lame because you can take it out with your fists safely...
ZOMBIE: it's a bit lame that they all look the same, but they're fairly effective actually, because they pack quite a punch, and you're often tempted to take them out by melee.
Z-SEC: lame because they look like generic SWAT despite being on Mars in the future, you'd hope at least they'd use old Doom helmets... The pistol guys at least have the decency to look like the Doom 1 sergeants. They're decent as enemies, but the shotgun guy seems out of balance, because he's both deadly accurate and fast-firing (uncharacteristic to player's shotgun). You'd easily lose 40% health from one of these left alive, while a submachinegun bullet would only cause you 2%.
IMP: I like the imps, I think they're better than their classic counterparts. Their attacks have splash damage, they're more durable and know more tricks. They're a bit over-used, but so are the classic imps.
COMMANDO: very tenacious soldiers to kill you. The tentacle monsters can be countered by you crouching, but beware not to get in their kicking range. The chaingunners are more annoying, because they die with difficulty and fire a lot. I think they were underused in the original levels (the chaingunners...)
PINKY DEMON: I think this is their official name, not just "demon" like in Doom 1. Their attack and speed are much better than before, just like in -fast Doom, but their roaring habit doesn't help them at all, because it just causes them to stand there and be dead ducks. It would have been better if roars made them heal. Also, why the fuck no spectres in Doom 3? It would have been dead easy for id to make a refractive texture material for a demon skin.
MAGGOT: they're very easy to destroy, but are quite agile and can catch and land you a hit before you terminate them. I kinda like them.
WRAITH: as said, lame. Their teleporting is useless, they just warp three meters forward, in your gun's muzzle, and they give you ample time of everything. Even their attack is easily dodgeable. Other monsters hit you near instantly (even the thin zombies have an instant blow in their repertoire), but it takes this guy a couple of seconds to land his claw on you. You can fist it to death.
TRITE/TICK: everyone likes them, but I find them trivial, and spiders don't scare me, so it fails. Three submachinegun blasts, and they're gone. Otherwise, they resort to regular Doom 3 tactics: go melee, try to jump on, try to dodge.
LOST SOUL: these things are small and low on hit points, but they're far far nastier than in Doom 1. Have four of these on your tail, and you're already fucked up. They're less dumb and more agile than their large older counterparts...
CHERUB: more durable and agile than trites, these babies are more dangerous, and often their waves never seem to end.
CACODEMON: LAME. Having half the health of old cacos and TWO eyes, they're nothing on the old beholder myth. At least they try to bite the player which thankfully is still dangerous... but even their fireballs are destroyable. Dumb.
MANCUBUS: Not very deadly, they only fire one missile at a time... their actual property is durability, they really eat your ammo.
REVENANT: Much slower moving than the old revenants, otherwise they're fairly similar. Their rockets do less damage and are destructible, but are fired much more often and are always homing.
ARCH-VILE: Lame. Their summoning ability is required, because they don't have bodies to raise any more, but it could have been faster, because generally I kill AVs fast enough not to let them do it. They have far too few HP and their attack is underwhelming. Note that a classic instant attack is still implemented in the script! But I believe it isn't normally happening, except in mods. I saw it in one of Kaiser's maps.
HELL KNIGHT: Well, they're the Kings, and they deserve being placed on the Doom 3 cover. Their attacks are actually deadly, they always do huge damage to the player, with a splash damage like the Cyberdemons in Doom 1. They're actually something to be careful when encountering.
VAGARY: Cool idea, but appears too rarely, and has too many hitpoints not to be a boss. The telekinesis is quite an unique attack, ahead of its time -- it took until the expansion for the Grabber to appear.
GUARDIAN OF HELL: Lame, it looks too much like a stupid console platforming-shootemup boss. And its attack style looks half-assed. Kill its beacons, shoot its vulnerable point, repeat until dead. The model is OK, however, good for modding I guess.
SARGE: It's a solid boss actually, who shoots you with a BFG(!) or tries to claw you doing massive damage. But is sometimes unfair and impossible to beat without cheating (I'm talking about custom maps here). One of the few id boss who rely on brute force, not puzzles, to be beaten.
CYBERDEMON: Looks awesome but has an underwhelming attack. Its rockets don't do as much damage as yours, and are quite slow... But its interesting characteristic is that it moves faster than you if you get tired [in custom maps], and you become a dead duck. Definite plus over the Doom 1 cyber, this part.