Arma 3 is a blast to play and practically the only option in milsim sandboxes.
It's absolutely unique in the way that everything can be player controlled, CAS, drone operators, Artillery, etc. Everything is or can be done by another player in your session.

Due to it being a sandbox game, most of your missions will be player made, giving it practically endless replayability. The modding scene is still active 6 years after launch and is still pushing out high quality content.