I genuinely think FarmVille is a contender, as I said in the other thread, but realistically gaming has existed long enough that picking just one is kind of impossible. There have been several shifts and revolutions. With how much of the revenue in gaming currently flows through mobile games, gacha games and live service games etc I really do believe FarmVille might be the strongest influence on the current landscape of gaming. But historically, it's possible Doom was more important for its development. Or even Super Mario Bros for putting home consoles on the map. I could even see an argument for Minecraft - it's completely ubiquitous and an absolutely global phenomenon.

Gaming is already big enough and has existed long enough that the question is fairly unanswerable. It's like picking the most influential movie. Is it Birth of a Nation for inventing cinematic language? The Jazz Singer for popularising "talkies"? Is it Workers Leaving the Lumière Factory in Lyon for being the "first"? Is it The Wizard of Oz? Is it just Citizen Kane? The truth is, it's none of them. It's all of them.