If you have Steam, then you probably received several copies of Dota 2 for free. I'm not a terribly big fan of the genre since it's just watered down WC3, but it's highly populated, somewhat harder than League of Legends, and less pay2win scammy.
Team Fortress 2 is another game that is still highly populated, extremely fun to play with people, and has a huge skillcap so you don't have to worry about peaking early. I'd recommend this game the most actually, since it's free to play and the default guns that you get for free are in most cases more well-rounded than the premium guns (which you can also discover as you play, albeit slowly).