What's the most annoying achievement for you?

[Vaelkder]

I think that it is a tie for me, between finally, FINALLY, getting the August Celestials Cloud Serpent, and Over Their Heads, from the Klaxxi Daily Quest "Dark Skies."

For the latter, I made sure that I didn't kill more than one or two of the mobs per run, and I didn't target any of the larger targets. I ran this for 59 straight days, but two of the listed targets (Amber-Shaper Un'sok and Dread Ambercrusher) just did NOT show up. Everything else appeared, day after day, ad nauseum, but not these two NPCs.

After smacking my head against this wall, I finally began writing tickets to Blizzard, asking if there might be a problem with the way that the NPCs showed up for this particular Daily. I received no reply, which is practically a given, but I did get Un'sok the day after the first ticket. It was another two weeks before the Dread Ambercrusher finally put in an appearance, with six more tickets being sent in to Blizzard.

[Terahertz]

2.7k in 3's
who even made this achievement

That achievement was so much easier to get in begin/mid/early end cataclysm. Now it's near impossible.

[CheeseSandwich]

I think that it is a tie for me, between finally, FINALLY, getting the August Celestials Cloud Serpent, and Over Their Heads, from the Klaxxi Daily Quest "Dark Skies."

For the latter, I made sure that I didn't kill more than one or two of the mobs per run, and I didn't target any of the larger targets. I ran this for 59 straight days, but two of the listed targets (Amber-Shaper Un'sok and Dread Ambercrusher) just did NOT show up. Everything else appeared, day after day, ad nauseum, but not these two NPCs.

After smacking my head against this wall, I finally began writing tickets to Blizzard, asking if there might be a problem with the way that the NPCs showed up for this particular Daily. I received no reply, which is practically a given, but I did get Un'sok the day after the first ticket. It was another two weeks before the Dread Ambercrusher finally put in an appearance, with six more tickets being sent in to Blizzard.

Get a cross realm pal or 2, have them check out what bosses are up on their servers every day, if what you need is up, get them to inv you, go kill it
Did that for over their heads and that one in the vale, One Man Army.

OT; As many here have said, The Seat of Knowledge is a pain that should not be inflicted on anyone. 6 levels of RNG for 10 acheivement points is horrible.

[Kazgrel]

AB Perfection annoyed me for the longest time being the last achieve I needed for Battlemaster. Also Iron Dwarf Medium Rare.

Also throw in the "don't take damage while doing this daily quest" achievements in MoP. I've gotten them all, but damn, I haven't wanted to throw a game control peripheral (be it a controller, kb, or mouse) in 15 years...then those came along.