The ACR is the best assault rifle in the game.

-Recoil is non-existent
-You can drop people within 3 shots if you aim for the center mass
-Iron sights are money

ACR>SCAR-H>M4>FAL>AK-47

I currently prefer the SCAR-H. I like the recoil and the power of this rifle. The drawback of this is the limited mag capacity. I do not use stopping power on assault rifles, the only time I enable that perk is for my sniper class.