ARMA 2 DayZ OverPoch, I know it's an old game, but there are still a lot of people playing and it's soo much fun with friends, because you can build bases, kill AI, gain coins.
ARMA 3 - Idk, there's a lot of mods to play with it.
Rust - Very fun, especially if you're playing with friends or clan.
Mount & Blade - Warband Viking conquest - really fun game if you haven't played it, I prefer the viking conquest one, because it has a nice storyline and stuff.