I too used the lightweight M4 in D1. I now use police M4 in Division 2. It is, in my opinion, the best combination of stability, damage and rate of fire, along with the ability to hit targets at range.
Carbine 7 is excellent as well, but always comes with a garbage talent so you need a near God rolled example so you can roll off the talent. Famas is outstanding too, but better in closer proximity in my opinion.
I like the Ctar, it's like a laser beam.
Among the low rof AR's, the tactical mk16 is an awesome all rounder. I've found a 0.7% shy of godroll one in last march and i still use it.
P416 also a solid contender, but i'm yet to find a god rolled one... or just one that doesnt take 100 hrs to optimize.
Police M4 probably if you dont have Famas or Ctar.
F2000 also there, the named Shield Splinterer especially good if you are lucky with the roll...but it doesn't have an underbarrel slot, but i guess you can live with that.
Honorable mention : Honey Badger.