Hearthpwn

Wins against: Mostly slow decks with very little burst damage.

Loses against: Boring decks. The kind of decks that contain only commons and the occasional obscure card that really doesn't have any business being there. 

Will continue and improve this as if I see better alternative.

Some noteworthy mentions. The neutral card that gives you paladin cards. Even if you are not facing paladins, you are still stealing from them.

Nexus-Champion Saraad can also be put in to allow access to class specific spells. Sadly this isn't stealing, and you will be hunted by both the players you stole legendaries from and the Pro-Steal-Police. Sorry!

Deathlord is a tech card in my Thief Priest decks to force people to go into mind control tech range. However with the new expansion out, people will be more than happy to flood the board with shiny, brand new legendaries!