Path of Exile
The best free-to-play game after
Dota 2. It's an isometric action role playing game with the look and feel of
Diablo II. It's technically an MMO, however you can turn off the in-game chat and will only ever see other players in game hub areas. While it's F2P, it's not pay-to-win, and any microtransactions are all cosmetic based and won't affect your stats in any way. Definitely worth a look if you were disappointed with
Diablo III and have played your way through
Torchlight II.
Counter-Strike: Global Offensive
I don't mind paying $3.74 to find out I still suck at CS. It's probably been close to a decade since I stopped playing this game but I still know the classic maps better than I know my home town.