Hearthpwn

Mastercard

Actually two, Baku and Genn.

For Standard its Genn/Baku

Wild it was Juicy Psychmelon. Card warped the whole meta around it and allowed combo decks to kill you on turn 3 as the biggest highroll. It warped meta so much around it, that basically your deck was aimed to stand a chance against Druid and then you are thinking about other classes as well.

the most impactful card of hearthstone is..actually two cards, Genn and Baku,  Standard is overrun with genn abd baku decks and most high tier decks in wild are even or odd

I would say the most impactful card in 2018 was Giggling Inventor before nerf it forced everyone to add counters in their decks just for that card and almost every deck run it .

The "Get Out Of Jail Free" card from Gadgetzan:

But also, in the real world, I would probably have to say either Leeroy Jenkins or Ragnaros the Firelord because I'm guessing they might have done the most hero kills out of any cards in the game, but that's just a guess.

There are basically three answers to this question.

1. The first card that made an insane impact and could be played in almost every deck was Dr.7 himself. Control, Midrange, Zoo and even some Aggro-decks... our lord and saviour was almost everywhere.

2. Remember the time even Priest ran the Pirate-Package? Guess who was in Charge. Patches the Pirate in his original form.

3. If we are looking at cards in their current state the answer would be Baku and Genn. It is getting harder and harder for new cards to have an impact in WILD. Never the less both Cards enable various top tier decks in WILD!

Baku and Genn obvisoulz and there is no other competiton in terms of impactful, cause these efect is for whole game so thats a blodz BIG

impact 

do you mind sending me a your decklist? i have been trying to make control hunter for a long time.

I would say Deathstalker Rexxar . It's in litterally 100% of all hunter decks in Standard.

Don't know about wild though, as I don't play wild.

 Jade Idol or Patches the Pirate. So impactful they nearly killed the game stone dead.

I keep cramping N'Zoth and The Curator in as many decks as I can (unconsciously), so I guess for me personally it are these two cards.

I don't play much wild, but from Dane's videos I came to conclusion that it's Sneed's Old Shredder. :)

Lich King deserves HALL OF FAME

Pre-nerf Corridor Creeper what? How is it not the strongest most impactful card ever?