There are quite a few that I'd vote for, but thanks to the Cataclysm a new leader has risen above all the others. And to think, it's supposed to be "cute" - turns out cute is the new suck!
Save the Wee Animals
You're supposed to run around a burning grove of trees and pick up any combination of of Panicked Bunnies or Terrified Squirrels. Question: With Night Elves connection with nature, you would think that they would know of some spell or encantation that would attract wildlife to their side, including these cute little bunnies and squirrels. But NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO! They ask some dude with an axe to gain the confidence of an already terrified little animal and allow themselves to be picked up and carried to safety.
Which leads me to the annoying part, picking them up. They are small, they move fast, their patterns are sorta random, sorta pre-programmed, and just when you think you have them where you want them, BAM! They quickly dart off in the other direction. They are suspicious little buggers, that's for sure. Don't they know I'm trying to save them? Just keep your cute little fanny on the ground and let me, the 6'4" man hero to lift you to safety already.
It's the capture part that gets to me, er, the rescue attempt. It's not easy to right click on a tiny moving target. It's so difficult, in fact, I need to call in help in the form of kobolds. Kobolds have these wonderful nets that they like to throw on people, so, in an effort to get this quest done quickly and efficiently, I enlist the help of 10 Kobold Miners who I send out into the burning wild to capture one critter each. When they are done I pay them with fake rocks, plastic painted like rock, and to date no kobold has complained.
I warn you, if you haven't quested through Hyjal, be prepared to be frustrated. The quest, even with it's cute factor, just plain sucks.
And that's all I have to say about that.