What card do you hate your opponent having the most?

[regen2k9]

I can't stand it when I come across black holes in gold/plat

[Feuerhose]

I hate Poison. I like to stall, so this "spits in my soup" as we say here...

[Kefka]

Explosion.
2 to destroy my permanents? 

[furballdn]

Explosion.
2 to destroy my permanents? 

Before the patch, it was 1 .

[Kefka]

Explosion.
2 to destroy my permanents? 

Before the patch, it was 1 .

I know. I remember when it was only 1
It's still annoying.

[UnderneathTheLens]

Pest. It always takes the quanta that I need.

[artimies7]

Pest. It always takes the quanta that I need.

Seconded.

[DarkStelth52]

FIREWALL. I have a burning hatred for this card 

[ElementalDearWatson]

I'm going to change my vote to Shard of Sacrifice. 

In the Arena, 6x Shard of Sacrifice + 2x Dexterity = 24 rounds where you can't hurt your opponent at all, or counter his defence.  All he needs is one Quantum Tower and one Shard of Sacrifice in his first draw and he can win the game before you've even had one single turn, if he's got some Heals or Shards of Divinity or the like.  It wouldn't be so bad if it were a Permanent with a timer (like Sundial) and you could actually destroy it, but as it is if your opponent's got a few of these, you might as well quit straight away.

It's even more annoying than Ghostmare.  At least with Ghostmare you could have  Towers or Sanctuary.  With Shard of Sacrifice there's nothing you can do at all.

[ralouf]

Actually did you noticed that SoSac make you loose 40 HP when you play it ? But yes a deck with 12 Sosac is helly annoying for any "normal" deck (with 30 cards I mean)

[inthisroom]

I'd like to answer "any card that contributes to beat me..."

But narrowing it down, I'd definitely have to go for SoSa, you can counter most other cards with various strategies, vs SoSa, it's Sundial, Antimatter, or just watch the opponent play for a while...

[FlareGlutox]

Not to forget Shard of Patience as a counter to Shard of Sacrifice.