It is without a doubt corridor creeper.
This. Some people saw some potential, but the viability in almost every competitive deck was not foreseen by anyone. It was basically a 0 mana 5/5 and played in aggro decks as well as control and midrange decks. Honestly, why should anyone skip a 0 mana 5/5 that is only in control mirrors more expensive?
Edit: caverns below is also a good pick. I didn't think about that card since I expected it to be good (in particular with 5/5 and only 4 bounces) but many people said it was too slow.