In my opinion the most annoying card is Conjurer's Calling. I can't even understand, how they could not guessed, that people would be outraged by this card. I don't even understand, why they made it a twinspell - card is broken even without it.
I also think that mage got too much freeze. But that's another discussion. What is the most annoying card in your opinion?
I'm stuck between Barnes, Spreading Plague, or Archmage Vargoth. On a surface level, they all look fine, but the decks they are used in and they reason they are used is what really bothers me about them.
currently it's Convincing Infiltrator no idea why they printed this card with how much people hated Obsidian Statue. This is the epitomy of everything a hearthstone card shouldn't be. It's boring to play in that it's just a minion you put on board, and has coin-flip RNG. Once they play it and summon it back a million times with Psychopomp, it's game over for any deck that plays a lot of minions on board. Minion-trading is what makes hearthstone fun and provides a lot of skill and variety, and any card or strategy that's specifically designed to counter that should never exist.
Any card that allow for a 30+ damage combo - Mechathun and the old mage quest are among my most hated cards. Nothing felt worst than seeing that quest pop up back in the day and just knowing that you already lost the match.
Arena: Conjurers (Jeez, I had runs, where that card carried me hard.), Discover cards which can lead into Hero Cards (well at some time when KFT was around and Rogue was able to get another class DK in arena..).
For me, it was always The Caverns Below. I always play expansions through. I braved through Pirate Warrior, fought every Highlander Priest, and faced every Odd Paladin. All those top tier decks that beat me, I still played. Un'Goro META was the only time I stopped completely. Once you saw that Quest, you knew the game was already over.