Treacherous Artifact ;)
Before Hungry Dead was nerfed, my Brigand with a splash of Undead-deck (I had Hungry Dead and Warlock) won almost every game it had time to play.
Now I mostly play my Alliance or Mage-deck for players I know are good. My Mage-deck's my favorite, as it plays out differently and has very interesting combos.
In the beginning I had an Unliving deck which had a win-rate of at least 95%, but it was boring to play, so I deleted it.
I actually had two more decks that won a lot more than some of my decks I have now, but now I've fixed my decks to what I find fun to play. It's frustrating cuz my stats could be a lot better, but when I get some of those dream-combos to brutally slaughter my opponent, it's totally worth it :D