I only used 3 for longer than 3-4 games, the Scar when levelling up early doors, the FAD which I LOVED ,but its unlocked too late, I actually got a MOAB with it on Downturn, hence my love for the gun, and the ACR. The problem with the ACR is that its noisy as hell, and the range sucks with silencer.
I dont use ARs that much, honestly becuase im a runner n get sick of getting blown away by akimbo merchants and smgs constantly. For that reason I use smgs 99% of the time. If I had to choose 1 though, id go FAD.