Either the protectors (first boss of Terrace of Endless Spring) or Mageara. Both of those bosses put me to sleep, they were incredibly boring. What's even worse is when groups failed at Mageara in, say, LFR, because the tanks are too fucking stupid to tank them in the correct direction or swap bosses even after explaining the extremely simple fight.
I like Galakras and at the same time dislike Galakras. He's not a boring boss, but he is a tedious one. I'm okay with it.
Heroic Blackfuse.....i hate that guy so much that I killed him and crashed his vehicle!!!!!
Sha of anger....he killed so many people back when MoP was released, and worse if you released spirit you lost your loot! He was horrible.
Kept telling people to 'ignore the raid leader, don't release or you'll loose loot'.
Lei Shi
Garalon
Paragons
Having recently gone back to complete older achievements, Heroic Dark Animus was the worst fight. When putting together a pug, you have to stop and explain the intricacies of the fight, and with the heroic cloaks on everyone, people accidentally kill an inactive golem with cleave, and things go to hell.
Outside of heroic fights, I personally hated Galakras because I sucked at assassination AoE.
If I can only choose one a tier...
Lei Shi
-The mechanics, random as they are, were annoying as hell. Also, she liked to vanish right when I had CD's up. My guild's Mumble was constantly full of cries of DAMN IT when she vanished, and not just from me.
Tortos
-The mechanics started off fun and got tedious real, real quick. Even though as melee I spent a good chunk of the fight on the boss, I ended up tanking bats as Prot for a while and...I just hated it. And the turtles. And...everything. Blah.
-Thok
Thok as melee is hell on Earth entirely due to Phase 2. It's no wonder so many guilds ran full ranged comps for this fight. Only so much I can do with Storm Bolt and Heroic Throw.
Hated Tortos the most. 24 people can be perfect but if your kicker messes up(and we had kicker issues for a ridiculously long amount of time)it's a wipe. Just so frustrating.
The 90% loadscreen boss.
Ambershaper, cause 1 person making mistake in fucking vehicle they've never been - wipe..
Durumu - dat maze that was invisible to me (eye problems) till they changed the graphics.. THen it became only slightly visible... Great idea to have purple fog with no distinctive borders on purplish/gray ground >.<'
Lei Shi because goddamit who even thought that'd be fun fight..
Sha of Sleep, cause sleep..
Immerseus.. Same as Sha of Sleep.. just sleep, but even more :/
Anyone saying anything other than Lei shi seriously needs to check their memories or accept that they weren't playing at the time
Lei Shi because it was soo damn annoying.
Durumu - While it was interesting experiment, the mechanics and way how the maze was colored was made it nightmare.
Only did SoO when it was current and the whole of wing 2 was wank.