Hearthpwn

To my side was launched with a complete spell hunter package and I remember playing that deck on day 1 of kobolds, and I definitely wasn't the only one. That card would hardly have been "discovered".

I stay with The Caverns Below. Everyone expected it to be by far the worst quest, but it quickly became the most dominant one.

Wasn't possessed lackey not that highly thought of? 

Not the worst maybe , but how aboutAwaken! And Devoted Maniac?

Spell hunter was once a tier 1 deck?

Quote from BrokenCycle >>

Not the worst maybe , but how aboutAwaken! And Devoted Maniac?

 Many comments at the time don't know will get a hero power activation in the effect and that is because many people think will be crap.

4 manas 2/2 rush and nothing more is terrible bad, trash tier card.

Quote from hillandder >>

Quote from BrokenCycle >>

Not the worst maybe , but how aboutAwaken! And Devoted Maniac?

 Many comments at the time don't know will get a hero power activation in the effect and that is because many people think will be crap.

4 manas 2/2 rush and nothing more is terrible bad, trash tier card.

 Really? I thought it was pretty well explained, with gameplay video’s showing how it worked etc.

Corridor Creeper was part of a card dump, and it won the trinity series like the literal day after.
I'm not even sure people knew it was released.

Caverns Below and To My Side! are the biggest ones I think.

The Dr. Boom hero card. Saw no play until they added a bunch of OP mechs in RoS.

I don't know about MOST OP, but the pre-nerf Bonemare was right up there with pre-nerf Corridor Creeper (which I think gets my vote).

The Caverns Below don't believe me? just search the forum for the reveal thread of the quest everyone was calling it garbage and the subsequent 180° the thread took once it got released

Caverns is probably the biggest one I can remember alongside its counterpart The Swamp Queen, the Hunter quest being the biggest flop. Nowadays it is really easy to see which one would be good and which one is laughably bad but most ppl missed both at the time.