Dota 2, Warframe, Team Fortress 2, Paladins, Unturned, Path of Exile. Statistically the most popular free2play games on Steam, in order of most played.
NOTE-NOT A STEAM GAME. But you should definitely try to get in the closed beta for Dauntless. It's going to be free to play. A friend of mine shared one of his free keys with me and it is such a fun game! Check it out!