Blizzard.
Think about how Starcraft shaped the RTS and brought it into the spotlight as a major genre. How Warcraft 3's hero system spawned DOTA and its numerous extremely popular clones. How Diablo and Diablo 2 made the roguelike RPG popular, revolutionizing massive online multiplayer. How WoW totally changed the MMO industry, and played a major part in the creation of the now popular free-to-play+microtransaction model because nobody could compete otherwise.
Battle.net was nothing short of the best online gaming network for years, hosting millions of players and matches of Starcraft and Warcraft, forging communities and economies of millions of player characters in Diablo. And it all started in an age of dialup internet. No doubt it influenced later services like PSN and XBL. Blizzard's role in the growth of online multiplayer gaming is enormous.
