Helldivers - has had over 15m players.

[G2ThaUNiT]

I bet! The game has remained in the top 50 of Steam's best sellers for 45 weeks straight now and as well as in the PS Store. Checked just now and it's at number 36 on Steam and number 49 on PS Store. Spent a lot of time earlier this year playing with friends. Such a refreshing multiplayer game.