Arma 3. I wish I could run this game better, my PC is too slow and I only get 20-50 FPS depending on what map I am on, what time of day it is and how many enemies there are.
I completed the other big campaign for Arma 3, Apex Protocol, just now. Apex Protocol was made with co-op in mind, but I played it solo. You unfortunately do not get any AI teammates when you play solo. It took me around 6 hours (vanilla campaign should take you around 15-20h). While it wasn't bad, I can't say that it was as interesting or rewarding as the main campaign, The East Wind.
Some guy modded Metal Gear into this game, lol. It can fly, shoot dual gatling guns, HE rockets, AT rockets and even has a cargo mode.
https://steamcommunity.com/sharedfiles/filedetails/?id=952560410