Sight: Personal Preference

Grip: Angled or Vertical Grip

Barrel: Compensator

Other: Green Laser 

My default loadout.

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.

Depend on your weapon. What assault rifle is it? I’d recommend the FAMAS, although many AKs are good. My FAMAS loaduout consists of:

Optics: Barska Elekro Barrel: Muzzle Brake Underbarrel: Vertical Grip Other: red laser

If you were to use specialist ARs for longer ranges, such as SCAR-L, L85A2 or AN-94, then considering using Magnified Scopes (between the magnification of 3.0 to 4.0), I'd recommend using the TA01 ACOG (due to it having good magnification and comes with back-up iron sights above it in case of a CQC confrontation, saving an attachment slot for lasers). However using normal Red Dot Sights are perfectly fine. 

In the case of the L85A2, using medium-powered scopes like the ACOG will need some help from Folding/Angled and the Muzzle Break due to it having recoil expected from a typical AR to balance it's ranged damage and firepower of the eqivalent of the AK-12. If it's not enough then use normal RDS.