For me it’s Benn’jahr, the Colossal a-hole. Honestly, he’s not so bad by himself, but in SR, he’s basically able to hide behind 4+ similarly tanky but faster bosses and be free to deny half the arena with indestructible bombs and multi-second stunlock at will.
He has taught me the necessity of stun resist, and I hate him for it.
Benny is a great sidekick for any other boss. Not so dangerous by himself, but he helps his friends to keep me in place. Seems to be a deep relationship with Aleksander, with the standard tactic: “You root him, I throw with meteors”.
My vote for first place goes to…well, it´s a shared 1st place.
Mad Queen:
Absolutely annoying in SR. Fast…and I have a lot of projectiles proccing, so I have to keep her on distance when fighting other enemies.
Garbagol:
I see him, I leave him where he is. No damage, till all other enemies are dead. I don´t want him in his second phase while fighting multiple enemies. Hits like a truck.
Reaper of the Lost. He always wrecks me when he shows up in Crucible or SR. I’ve never gone hostile with Barrowholm to face him in the campaign, so I have no idea how to fight him effectively.
Grava’thul, obviously. Especially on melee fire/lightning builds like Warder, Commando etc he’s a true pain in the ass.
He has too much annoying abilities.
Mad Queen in high SR (75+)
This.
As I play mostly melee builds I hate this dude and his nullifying instagib that is not always easy to see and react. The main reason I switched to low FX - to see the movements of enemies behind the flashes of any flashy build and Grava move is one of the most dangerous one.
I hate that moosi is the only nem left behind after Zantarin got some rehab.
I don’t hate any particular boss but i absolutely hate the speed of some of them (like Reaper or Korvan). 1) It looks stupid 2) It makes the enemies stutter around the obstacles 3) It shits on a big portion of the game when they appear - positioning. How are you supposed to roleplay as a ranged character when the enemy closes the distance from the offscreen in 0,5 sec? What was even the thought behind this ultraspeed mechanic?
Aethersnek. Combined with anything else is pure cancer.
Also Gravathul’s nullbolt makes him the only boss in game to bypass any player defense.
Kubacabra. That wily bastard as a nemesis just makes me rage. 
I’m also not particular fond of that hero enemy in the Plains of Strife who sneaks up on you and debuffs you (can’t remember the name.) I hunt her down like a rabid raccoon just out of spite.
You mean loxmere nightmage?
Grava’thul’s debuff and mana burn sucks, Garbagol’s 2nd phase is pain in the ass and Fabius can two-shot me, but nothing I can’t handle.
Getting Mad Queen and SR (66+) is over. She doesn’t kill me in the first place, but there is no way getting her health down. One wrong step and she is at 100% life again.
I had her once in SR 77, at some point realize that I am not doing enough damage while running to reduce her health, so had to risk more. Anyway 6.30 time killing her kiting, people finish Gladiator run faster 
Grava… And by extension every hero that dispells your auras. Grava is the worst.
So I make it a habit to get the fck out of melee range when he tilts his head and do his firebreath, may or may not take out his null bolts to hunt you
Valdaraan in SR.
Oh so you want to aggro the boss one by one, lemme teleport you in the middle of the room.!!!
Grava is one of the worst designed bosses I’ve had the displeasure to deal with in a long ass time and I play a lot of shitty games with shit bosses.
You take away his Grava balls and he’s not really a threat. I recall him having more “slow” abilities and his grava balls weren’t telegraphed in previous incarnations. He’s also more of a pain in the ass for melee builds etc.
I don’t get the hate for Grava, he is awesome 
Grava’Thul isn’t so bad either, never had much problems with that guy. His deadliest attack is really easy to dodge, well at least when Benn’Jahr is not around. But that guy is slow so I just keep a distance and hope for the best 
Kaisan on the other hand, he’s fine in campaign (as all bosses are imo) and in normal SR chunks he isn’t too bad but I really hate when he spawns in those tinyass rooms like the portal one. At least his bolt attack was toned down recently, just wish his crystals would go down quicker.
edit: almost forgot Valdaran, his teleport spamming is really annoying, at least in SR. But Kaisan takes the cake.
I don’t have a problem with his dispel balls. It’s almost everything else that I dislike. Tracking fumble pools is just a dumb mechanic (and so is fumble while we’re at it). AoE pools are for stepping away from so you don’t take damage/are affected, not for “hey you did the right thing by getting out of the ‘GET OUT OF HERE’ area but we’ll penalize you anyway because fuck you” so you just facetank him and don’t even bother to get out of the pools because it’s more trouble than it’s worth.
The wing flap attack having tracking is just lazy. If it’s a cone/straight line, allow me to actually reposition properly to avoid it by sidestepping and such, instead you can just run away and he just cancels the animation and chases you down so again I just facetank it if I can deal with the debuff.
Then there’s the whole %HP reduction aura and all the %HP damage. The whole boss just comes off as extremely poorly designed to be threatening. I’ll just continue to facetank him though, I guess 
And yes, if you’re ranged you just get to mostly ignore his mechanics, but that’s just business as usual.