Sights with magnification between 1.5 or 2.5 should be a safe bet (Doesn't apply to weapons like the AN-94 or other specialist ARs)
Grips: Folding or Angled
Barrel: Muzzle Break or Compensator should work (depends on the AR's recoil)
Others: Green Laser or AP rounds (if you're struggling with hitting consistent headshots at range with an AR to reduce the TTK, doesn't apply with SCAR-L or L85A2 or any AR that could 4SK at longer ranges)
That's my M16A3 loadout. Works wonder. Compensator if I'm using AKM.