If we are talking about bitcoin Airdrops, then the 19700 BET from Gavin is one of the biggest in bitcoin history. But we are talking about the old day when the bitcoin price was lower than $1.
I would say the casinos have apport the biggest "BTC airdrops" in the past years, Just think about how many people have claimed the faucet on Freebitco, or how many people get prizes on the casino, or tips in the chats. I remember in 2014 how the casino called Luckybit used to have a nice Chat Lotto, where people post their address and the chat lotto sends BTC direct to the user's address, the normal prize was between 0.005 - 0.001 btc, and in the happy hours they pay crazy lottos up to 1btc.