Most annoying monster.

Hydras aren't that bad, you just have to fight them properly. They fight melee, so just get a tank to get them in a group and have an elementalist nuke the bloody hell out of them. Sure, meteor and fireball are annoying, but with a well formed group a monk or two can keep everyone alive long enough to finish the hydras off.

Of course, things that come out of the ground in large swarms aren't that bad either... in fact they're quite easy usually... but they're so incredibly annoying!

Wurms, when they get stuck in an endless knockdown chain.
On occassion, they'll surface and burrow rapidly without stopping, and anyone nearby gets knocked down over and over and over and over again until they die.
The knockdown damage might not be a lot, but when you can't do anything about it, it becomes problematic.

When it doesn't happen, I can deal with them, but when it does, it really manages to piss me off. It's times like that where I really wonder what the heck ANET was thinking.

When soloing, i hate DAMNED CLERICS. Worse spamming of RoF i have ever seen.
Skeleton scorcerers and their blinding flash spam, argh!!!

Stone summit gnashers and the constant raising of minions are bad too.

Overall, the most hated enemy of mine has to be sand elementals. Knockdown and blindness combined is so annoying.

Yup. Wurms get my vote. As if that endless knockdown cycle they do isn't bad enough, there's often minions around them that will take advantage of your perpetual tripping and maul you.

Azure Shadows are what I hate the most.

Dolyak Master - So freaking annoying
Stone Summit Arcanist - That stupid hex they cast that gives you 50% slower movement and health degeneration. Impossible to run by these @$$holes.
Anything that comes out of ground - MAINLY Azure Shadows but also the Wurms when they keep going under and back up cause you take damage and fall over. Ugh, so annoying.
Hydras cause even with sprint they keep pace with you. Ditto to Sand Drakes.
Whiptail guys - I'm sorry but the idea of a scorpion like thing casting curses, setting traps, and shooting something out of their tails just bugs the hell out of me.
Scarabs - Trying to do the switch to monk profession quest has developed this hatred of mine.

Quoted For The Truth

Jade Scarabs. Chiblains = no enchantments. And they have a couple other hexes, and always attack at an absurd rate with amazing damage.

2 Jade Scarabs > Me

Sand elementals. Definitely sand elementals.

My vote would have to go for the packs of Mindblades in the Underworld... ugh

*looks at other forum* so that's how gwen is going to be implemented during the comic books!

All of the ones that pop out of the ground are annoying. Azure Shadows are the worst, if only because they're so common late game.

Any monster that loves to give conditions back to me... man they piss me off. Jade scarabs own the hell outta me... I swear it's like they wait until I set traps for some minotaurs or some other mob, then they boil out of the ground, go out of their way to set off my traps, and then give all the bleeding, crippled, burning, blindness back to me. God forbid I poison them. Sand Giants do this as well - they can just keep handing conditions back to you. At least with Sand Giants I can condition them up and keep a distance... those jade scarabs are marathon runners.

Although this little quirk has paid off... Stone Summit Howlers will attack at range unless you put some condition on them - as soon as you have put a condition on them, without fail, they will charge their stubby little asses up to you to use Plague Touch on you. So, when I see a Howler, I plant all my traps and stick him with a Poison Arrow. They come huffing and puffing up to you and then <SPLAT>. Pure comedy I tell ya.