While we don't know which is the worst, we know many that aren't.
Lead Designer on Hearthstone, Ben Brode, recently gave an interview with Yahoo Esports about One Night In Karazhan's reception post-release. In it, he mentions a low win-rate card which triggered a reaction from the community who is once again trying to figure out what the "worst" card in all of Hearthstone is.

Seven months ago a thread was spawned as players tried to figure out which card it could possibly be. But Brode now says none of the cards listed are it, "It's not Majordomo Executus. There was a thread on this a while back where people tried to guess, and while there were hundreds of guesses, literally nobody guessed correctly."

Reddit user czhihong put together a list of all the guesses which Brode confirms does not include the terrible card.

"Yes, it's not on this list. I am going to stop answering though because eventually people will just list every card and stumble upon it by accident," Brode says, "and I really believe people love playing the card and would be sad to realize how bad it is. It's possible it has been dethroned since I last checked."

Here's the list of debunked cards. Any guesses?