With the sole exception of the V, former commandos are my #1 hated enemy.
Nightmare arachnotrons either appear on N! or on too low of a depth to be a problem. Normal arachnotrons are a joke in melee and are still 1d5x5 at range. Mancubi only kill you in one hit if you're unlucky on the terrain and 80% move speed, and Revs can do heavy damage but can't ohko you and are still fairly easily dispatched. Hell Knights come close but only because they appear so damn early (level 3 really can i have cacodemons pls) and can really threaten you without those ranks into Finesse or hellrunner, they also have 110% move speed so you can't reliably lure them on challenges.
They sport 42 max damage, which is more then every monster except for the strongest versions of the elite formers (SSG sergeant, Minigun captain, napalm commando) and the Spider Mastermind, which flinchs. ALONGSIDE this, they also have +3 base accuracy (which basically translates to 100% on UV/N!) which is like, give me that accuracy for MY plasma rifle.
Yeah, these things are why I drink my haterade. Them and anything below level 5 on AoB.