Insurgency has good coop potential, but could use more open and larger maps (although not easy on the engine).
Day of Infamy is alright, same thing, maps are too small, engagements take 15-20 minutes max.
Arma 3 Escape the Island can be fun, if your team mates don't run around like headless chicken. (This has to do in parts with the map missing player locations -> can be easily fixed with a custom script.)
Rising Storm 2: Vietnam is a meatgrinder and has no proper bot support, but could probably be modded by a few people who know UE3/UDK well.
Fog of War could some day become good, but is still in early access.
Operation Flashpoint: Dragon Rising Island War / This is War modification can be quite fun. No dedicated server support though. And i think a hard coded maximum of 4 coop players. Versus mode has 8, iirc, so maybe it can be circumvented somehow.
Then there is Insurgency: Sandstorm on the horizon, but it's going to take a while still. I think i read somewhere closed beta is going to start at the end of the year / december 2017.
And, of course there's the old stuff:
Rainbow Six / Rogue Spear / Raven Shield, Delta Force 1/2, Ghost Recon, Swat 3/4, Joint Ops, Hidden and Dangerous 1/2.