I nominate Copper and Estate
I did a bit of research in Council Room, and poured over masses of logs, and we all know how important the early turns are to get momentum!
And the one thing all these top players have in thier decks around 4 and 5 are Coppers and estates! I didnt bother looking at the earlier turns,because they had so many of each of these they must have picked them up right near the beggining. Im surprised not to see it at the top of the Level 7 openings to be honest