M16 and ACR at the beginning. AK replaces the ACR when it's unlocked. I also use the Mk14.
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.
I use the M16A4/RDS/Rapid Fire (my signature gun/loadout) the SCAR-L, the M4A1, the MK14 (first MOAB with that one) and in small doses, the ACR.
Other than those, the other assault rifles dont interest me as much. And i hate the AK47. Preference i guess.