I was going to say vortices but they don't usually spawn together in large clusterf*ck packs of doom :(

Any mimic, followed very closely by those stationary skelly druj mage things.

My vote goes to Gravity Hounds. But, that's just me. :)

No, it's not.

Hounds show up on telepathy, though, so can be easily avoided. Vortices don't.

Never understood the hatred of magic mushroom patches: they're easy enough to ignore, since they can't run after you.

In fact there aren't really any monsters I hate. ToME water hounds, I suppose, since they stun, destroy potions, and cause significant damage, and generally appear on watery, and hence open, levels.

My least favourite / most hated unique has to be the Mouth of Sauron: spells powerful enough to kill many characters, and if you let him get to melee range he has disenchant and drain-charges attacks.

Hm, it depends on depth.

First nasty ones are basic element hounds because they can destroy (expensive and useful) parts of your inventory or damage your armor.
Well, I just tend to ignore the armor damage and struggle my way to deeper levels since soon enough acid resistant armor will show up.

Then there are some early uniques and monsters that can disenchant which really can be painful if a powerful weapon gets disenchanted.
All the undead that drain stats are similiar naughty.

Later on when you are powerful and may have resistance to disenchantement and even immunities to some elements you have to fight monsters that drain charges from your precious speed and healing staves.

And the game has monsters to attack you with powers beyond any resistance (most hated are plasma and gravity) or where resistance is very hard to get like nether (which can cause immidiate death).

Even with hold life and sustain stats you fall victim to time breathers.

What a cruel game! ;)

I see I have so much to look forward to. I have been more successful lately then ever before and it seems that success will be rewarded with more monsters I will avoid like the plague! :)

Black Reavers. Won't even go near them.

Filthy street urchins. I fancy a quick trip to the general store for some apple juice and I get jumped by a million and a half of the little cretins. Thank goodness for ball-spell activators.

Those darned 'Z' hounds. Nothing else causes me more aggravation.

Also, time vortices. "Time" is such a cheap mechanic. "So you worked hard to get all your sustains, and hold life? Well, they DON'T work!"

Also, Time Hounds, because they're an unresistable disenchant away from being the Perfect Storm of "cheap".