That depends on the mission. I use Community Faction Project and several other mods, but as I am testing in vanilla most of the time, Syndikat or Looters from contact often make up the enemy. One of my favourites is looters v Spetznaz playing as the looters.

Dear Alky Lee,
Thank you for your mod on a new map, I love it!

I have a question: what is your main & favorite mod for opfor faction during playing HWS?