This is a hard one, so I'll just make a list of some annoying bosses:

Archimonde (Ppl always fail at the retardcheck
Gurtugg Bloodboil
High Warlord Naj'entus
Tiger Boss in Zul'Gurub

Now I'll let you continue!

Gurtogg indeed..

Solarion its just retard check all over again and ppl still fall asleep same goes for Archimonde ppl sleep when they play "some times there is fire mixed up with -"omg lag spike omg i clicked on the tears but they did not worked" and so on" i rofled every single times i've heard some of those

Haha @Drusalnik & HeuF

twin emperors

i find twin emperors rather fun :P but ill have to agree with people failing retardcheck on archimonde and gurtogg to be the most annoying ^^

Tiger Boss in Zul'Gurub

Essence :)

Were stuck on archimonde we got council / sharaz and all in hyjal but archimonde wy is he soo damn hard >< his fire sucks to

we have killed archi with a PUG raid for the past 3 months :o

bloodboil is fun :D

retarness is hard, this have been saw in our karazhan IMBA difficult boss, ARAN.
but afterall i think most annoying boss have been maybe Alar :D

well thats5 you see my point killed those zilion times but there was not a single time where some one did not failed from lets say 30kills on archi my old guild before the disband had like 5 or 6 1shots every other time some one dies on fire or dies from "omg i pressed the tiers they did not work" ......... so yea i think those are the most dificult and annoying bosses for alot of ppl :) for me its a great fun all in all ;D

twin emps in aq40

Archimonde!

Tiger Boss in Zul'Gurub...

archimonde & M'uru(pre-nerf)
in 3.0 M'uru is nothing else than little bit of DPS

Archi

twins

Tiger boss

i'm a rogue, okay? so, i hate that Reliquary of Souls bi%$* ....and - Heigan. I cant dance this shit, its annoying, just die freaking witch doktor! (i love my memories of good old Naxxramas - step right, one more time, and again and stop, now step left, yeah right, once more and ups, the 4k acid wall hit u)

Being a horde rogue preTBC, i would say sartura was quite annoying to learn. timing those stuns, and getting in there with our crappy dmg reduction, hoping to dodge whirlwinds to get kidneyshot in there. Lucky alliance had paladins who could hammer of lol when needed.

in TBC i've been playing lock mostly when raiding, and i would say its a hard choice. Archi is quite an arse. RoS as mentioned over, was imo quite fun. nothing says love like 19k shadowbolts, which means a 9,5k backfire in my health.
I've been sitting here thinking about what boss in tbc that was most annoying, and nothing really comes to mind, meaning that Blizz improved alot from vanilla -> tbc in boss design imo

pretbc Viscidus/Loatheb

Viscidus for being heavily bugged most of the time and a fuckfight to do otherwise and loatheb for all those retards who cant remember a rotation of 6 consumables and wipe u , then ur back to SM farming grave moss.

TBC

Prenerf vashj ,illidari council and prenerf muru phase 1 PUSHBACK ON MINDFLAY !111111111111111 nice one blizzard , also gotta love phase 1 wipes due to dics/random mistakes.Although its a great boss , muru can be quite frustrating at times.

Chro/Vashj/RoS

Pre-nurf M'uru no doubt! I don't think I've ever spent as much consumbles progressing on him on any other boss.

Twins , Archi , RoS !

Archi!!! Grrr

1. Archie even tho i love the fight...but ppl still fails at using tiers and doesn't know Fire= Bad
2. Na'jentus there is allways someone popping the spine too early and whiping the raid grr...
3.Mother sharaz when ppl run back to they camp with the debuff on ...grrr

well hmm 5 maning vc, with lvl 18's. omfg there so anoyying when healers just dps and they be like what heals?

M'uru -_-, so much raid dmg, pain in the ass

Archimonde .... is pissing me off :)

Muru

Vael for sure. After some try a rogue go to its side, he get aggro, everyone die. And then he ask "what was happened? i didn't speak whit it"

1) Veal
2)Archimonde
3)Priestess Delrisa at MgT HC
4) Na'jentus
5) Nefarian (Priest call @ 20% happend me about 6 times in my wow carreer :( )

Pre nerf Muru and Teron lol

I'd say Netherspite and Illhoof were quite annoying back when Karazhan was serious content. Otherwise i'd go with Lady Vashj, Kael'Thas (The Eye) & Illidari Council.

Malacrass when the zul'aman patch came, since we didnt have any aoe healers, only 2 karazhan geared holy paladins ^^

But actually it's not annoying boss. Every one know him. Easy boss.
-First time I'm MT for High King Maulgar. MY Unbuffed HP was only 12344k (buffed likes 14,9k). That time was our MT has 1 shotted :( & I taunt that Maulgar. That raid was really annoying. So many guys was running for zone out & some healers spam me on heals. That's really funny. I was told in vent (Come back noobs hurry). & that runners back to place. haha. But Maulgar's hits like 6k on me lol. I won't forget that time. (Lol almost all healers focus on me. but 7 healers hehe) But I remember my fingers dancing in keyboard lol. (Heroic strike, shield block, devistate, shield slam, revenge ...etc)

But I've never been in Sunwell plateu. Becouz very busy for my class.
But I think , for me A'lar was really annoying. I'm taunt adds then give Pala tank. That was really annoying :( always run run run run run run run taunt , kite , run to pala tank)

kel'thuzad. it's cool because it's the last boss of naxx, but i hate being 5 min killing mobs like a nerd until the real boss starts xDD

An annoying boss would have had to have been Prince in kara. Even though he was easy those random infernal drops always had to be really inconvenient didn't they.

oops i replied to Ganker >.

Mu'ru or 4hm pre-bc

Gotta love retard checkers as Heigan and Solarian, but for now i say the boss thats most annoying have to be Gurtogg and Twins (In Aq 40)

Thaddius 25man with the naxxlag...

i hated chromagus as a horde priest @ 60, nothing but hiting decursive and chain poting becouse dispell was ( is ) so fucking expensive. maaaan did i envey the paladins

Drusalnik & HeuF

Archimonde

Where is left?
W8 that is right....:|
U nub u wiped half the raid.....
Sry guys, it's ok u can do it, he only has like 30M hp right?

Fail ppl that fail at Thaddius, because it doesn't matter what class u are, if u have the best gear or not, if u are awake or not, u just need to know what way is left and what is right :| bah

Pre-nerf Muru hands down ...