I’ll put a few edits in here for you, not taking anything out:
nax: Undertaker was the most meta defining card of the expansion pre nerf
GvG: Dr. Boom was the most played card of the expansion, and really became the most recognized and played card, as it was in almost every deck at the time.
gadgetzan: Patches the Pirate was maybe the most played card of not only that expansion, but I’d be willing to bet it’s played rate made him a top 5 most played card the entire time he was in rotation as pre nerf patches had charge, and made any 1 drop pirate the best one drop in the game Jade Idol also was a card that vastly changed hearthstone as we knew it. Never fatigue and infinite Jade generation forced almost every real control deck from the meta(making pirates even better).
I don’t think you could write the history of HS without those 4 cards, and I bet a lot of people would rate all 4 in their top 10 most hated (or loved) cards of all time.
very good additions, it is pretty tough to remember how the meta was back then, most of my listed cards are a bit biased towards their use in today's meta.
also in the first 2 expansions the meta was heavily comprised of basic and classic cards like sylvanas, ragnaros, and other epic and rare cards that got hall of famed.
Miracle rogue is probably the greatest example of how it made good use of basic and classic cards such as gadgetzan auctioneer, edwin, leeroy, throughout many years and many meta shifts it stood resiliant for a long time.