Now, it would definitely be Riot Games.
As player, spectator and developper, I'm so impressed by their work, the evolution of their game since the release in 2009, how they listen their community [1], try new things and, most of the time, admit their mistakes (e.g. removing the ranked solo queue in 2016) and rollback [2] or improve their changes when it's needed.
Their game is the most played game ever. ~100 millions active players every month [3].
Their game is also the most watched game ever. ~40 millions unique viewers for the finals of the last World Cup in September [4]. Hundreds of thousands of viewers on Twitch every day (without even counting the Asian region). They have revolutionized the "e-sport" in Occident. I don't play anymore (for now) but I'm still watching most of the professional games because they created an accessible, passionate and entertaining game to watch (for those interested, the new season just started in Korea, China and Europe, and tomorrow in North America [5]).
Really inspiring. Congratz Riot Games.
[1] https://www.reddit.com/r/leagueoflegends
[2] http://na.leagueoflegends.com/en/news/game-updates/competiti...
[3] http://www.riftherald.com/2016/9/13/12865314/monthly-lol-pla...
[4] http://www.espn.com/esports/story/_/id/18221739/2016-league-...