League of Legends
LoL is the big brother and the most played game by far. With micro transactions involving skins, champions and different things the players become very involved which lead to more money for the company behind the game, Riot Games. This means that they can hire a great amount of personel such as developers to make the game even better all the time.
If you have any questions and want to learn more about the game, I suggest you head over to their official site here.
Valve also have their successful competitor to LoL in Dota 2. The game had a very long and extensive Beta-testing period with a lot of players and interest. Now when the game is released to the public it keeps growing and starting to see LoL over there in the distance.