Ghost recon wildlands has the most ease of access co-op there is. Simply a case of you login, see your friend online and hit one button and join the game.
When you join you can each select different missions you are free to go off and do your own thing but when you select a mission it will pop-up on your friends screen and they can either agree to sync up the mission or carry on with their own thing, even better if the mission comes up as white it means you have already completed that objective but you can still join your friend and redo the mission.
It's honestly one of the best thought out co-op experiences there is, the only bummer is that when you die you have to bleed out, I don't think you can rush the count down, been a while since I've played.