What card do you hate your opponent having the most?

[Onizuka]

SoSac.

:|

[Calindu]

Sanctuary and CC.
I hate to see my discord failing and I hate to see my creatures pooping out of the field.

[Cel]

I think I'll agree with almost everyone else and say Dim Shield. 

[RagidyRock]

Ice Bolt/Ice Lance. <_<  Why must this card ALWAYS FREEZE ME with one hit?  Just... why?

[darkrobe]

devourers, i run tight quanta decks generally and these suck. to a lesser extent discord or black hole, depending on the deck.

[Zeru]

Every single card my opponent plays is bad. My opponent should feel Quake, Discord and BH and lose the game without playing anything!

[DrunkDestroyer]

Discord.

It makes no sense to have one of the strongest cards in the game against monos and duos.

Likewise, Supernova, although there are some exceptions to this rule.



(On the other hand, cards I like my opponent to have include Dune Scorpion, Aflatoxin and Mindgate)

[Daraxyl]

Rewind!  Especially on my rainbow w/ tightly controlled quanta. Gets rid of my creature for several turns AND prevents me from drawing a new card.

[rosutosefi]

Rewind and discord. Discord is the embodiment of epic trolling.

[Pkmn31337]

Rain of Fire, Procastination and Pandemonium ruin my day.

[FlareGlutox]

Surprisingly I'm the first one to say Otyugh, especially the upped one.

This is the case, because I really like decks with creature-spamming, namely Pestal (my favourite deck), Rol/Hope, or recently a deck featuring Rustlers and mitosified Cockatrices (there are some others as well, but these are the ones I use most frequently).
A single early Otyugh can annoy any of these three like hell, which makes it pretty important to get Eclipses or Electrocuters out to counter it, while the poor Rustler deck can only try and outrush the opponent.

Edit: Ah, wait, furballdn allready had it in his long list, I failed to read as a whole. But still, I'm the first one to mention Otyugh as my single most hated card 

[Time_lord_victorius]

Quick Sand, nuff said