As player I only don't like Pain Elemental, especially when this bastard is not restricted to 21 Lost Souls, but as mapper I love it, because it's so easy to place it somewhere and make player dance around to eliminate this bastard as soon as possible, lmao, makes gameplay in my maps more dynamic, so, yeah, we have difficult relationships.
Generally, I'm also not a fan of Icon of Sin, but it's mostly the matter of right implementation and me, who don't like to hurry in Doom maps.
Also, Spider Mastermind is very lame as a boss, because of its Wolf-3D hitscan nature, but at least you can kill it with a couple (sometimes one) of BFG shots.
Maybe I'm the minority, but my most favorite monsters are Pinky and Specter, because they're so freaking cute, :з
Just look at this pink fluffy boy, isn't he a charming?
As for other monsters - I think they're mostly good and have their usage.
That's why Doom is ETERNAL
(if it counts, ghost monsters bug sucks ass when you're a mapper)