Kestrel and Wakaatu are fine to list but you will be labeled a coward if you aren’t Koty or Ultra.
On that mark I’ll start with Sire. I don’t find Sire difficulty by any means but I think its bloated HP and its bizarre head hitbox annoy me enough to not enjoy fighting this enemy on anyone but Marksmans. They charge aggressively to just to annoy you.
Close second would be either Lancer Palace Guards or Ice Scions. Palace Guards have insane dmg with any weapon but the Lancers fire rate makes them so much fun. Ice Scions are just cheaters. They can freezer faster than Gunners Flash Freeze card on Wave 1~4 frenzy.
Silent Poppers and Sires are some of the worst kind of enemies overall. Not having any audio cues for whatever reason for these enemies makes them more irritatingly obnoxious than they already are. It only happens on specific maps too. Rail Line being the most notorious map. Peace Makers feel like cheaters too sometimes lol
I honestly think they are slightly worse than a lancer wielding Palace Guard because their enforcer will down you faster than a lancer. Especially from up close. Just like you said, their range with the enforcer is insane. Almost like a laser beam. Feels like they have that assault rifle accuracy card equipped from Gears 4
Boom Scions. I hate when they one shot you point blank while I’m face hugging them AND in my ult. They just refuse being shocked.
Another pet peeve of mine is Cyclops. Those sneaky ■■■■■■■■ are always chainsaw ready and waiting for you around the corner.
Rejects, Leeches, Juvies. The latter two are literally just annoying and nothing but, while the only thing Rejects are good for is easy ult charge or free Nomad chainsaw kills.
You may be surprised to not find Boomshot Scions here, but they at least provide a level of entertainment when you get to slap them instead of them slapping you. Figuratively, seeing as they seem to prefer shooting their weapon point blank instead of meleeing. If they tuned the damage down a little and made their Boomshot consistently cause only a single explosion, they wouldn’t be half as bad.