ARCO as of now, even though the RCO/MRCO I prefer and I think are highly superior for close quarters, the top scope on the ARCO (IMO) sucks dick, but the top scope on the RCO is amazing, and the zoomed out perspective on the MRCO (especially with an MK1) makes the gun feel like it has no recoil, at least for me.
And yes, I played back when only the MRCO and ARCO were accessible to rebels and the MRCO fits the profile of the MK18 better, so I used it, the ARCO just feels derpy on that gun.
So... ARCO for Mid-Long range, RCO for CQC, MRCO for shooting people out of cars/helis.