Mirror, elixer collector and rocket.

Mirror takes out the entire fun of knowing how and when to counter a card, and trying to save counters by working around it using other cards. And also the other way around, baiting counters to a card to then place it. Mirror makes rhis way easier and is simply annoying.

Elixer collector is just a card that can either have extreme value, or not, but it never has no value, because the opponent has to react with a spell or miner, which both are of almost no counterpush value. Furthermore it just makes the opponent unstoppable when you dont have something against it. Yet it does fill a spot in your deck in which many people would agree could be a better card.

Rocket is just destroying about everything in beatdown and xbow decks, making the most classic playstyles less and less playable. Rocket cycle, sometimes paired with mirror dominates the meta and its just plain boring to play with or against. No rocket would probably mean sparky would be op tho.

A close fourth would be freeze, yet that card does require more specific decks and isn't always of value. Yet I do think its annoying.