What card do you hate your opponent having the most?

[ARTHANASIOS]

  DEVOURERS, I hate them so much I want them banned!
  EARTHQUAKE, it is really annoying when you have 3 stacked pillars and they'er gone in an instant! Don't forget that the Earthquake is the reason most mono-deckers replace 6 Pillars in their decks with 6 Pendulums of the same color!

[regen2k9]

SoSac.  So hard to successfully counter within a game.

[Tymalous]

Miracle.
It doesnt matter what deck I'm playing, if I see multiple miracles in a deck I just quit. Either they deck you out, or you waste your sosac or shields, and then they kill you.

Other than that, reverse time is imo one of the most annoying cards ever. Not only do you have to pay for that creature again, but you can't draw any other cards.

[semibalanus]

Its already been said, but rewinds, in combination with nightmares in ghost of the past decks are so annoying.  works great but I am not a fan.

[eaglgenes101]

Repeatable PC (Pulverizer, Butterfly effect).
One deflagration, and I get another shield out. One of these, and the rest of my shields and hourglasses will be crushed soon.

[furballdn]

Seeing plenty of EQ and BH in gold these days. Very frustrating when you're running a rainbow. (mono decks laugh at the BH though, not so much on first turn or second turn EQ).

[Anarkhizt]

Bonewall in death swarms...cuz unless you have momentum to get past it...just give up cuz by the time you get thru all the additional bones, their creatures either got you down to 2hp, or they big enough to eat you whole... 

[semibalanus42]

I hate seeing sosac and also nightmares

[AlterDevil]

Dimensional Shield and Immortals >< It's like sitting there and getting punched repeatedly.

[Time_lord_victorius]

SoSac, Morning glory and flying weapons used on Morning glory

[McSod]

Havin' played quite a few more Instosis matches... I wish to punch Explosion in the face. Hermes... Oh, how I loathe thee.

[valiance]

Antimatter. Followed closely by black hole.