None as Riot Shotgun with both perks stomps all three.
Totally smokin crack!! The LMG smokes the Riot Shot gun by a MILE! The LMG smokes the Assault Carbine. Both guns get benefits from the Grunt Perk. Not even a comparision if you have actually used both. The LMG is even better then a minigun, because there is no delay in spin up. No tiny magazine like the Riot shotgun or auto rifle or Assault Carbine. Only two guns are better for a run and gun playthrough in my opinion - the GMG and Mercy
LMG with AP rounds chews up anything with DT in the game. Go take a group of 5-6 Deathclaw with the Riot. No cheap sneak kills - just straight up run and gun. You will take a couple of hits, not with the LMG. Cazadors, Yao Gui, Geckos, Feral Reavers then use HP rounds - the DPS becomes amazing! Who cares about knockdown from Stay Back when they are already dead. 5 HP rounds will drop a Cazador, 8 will drop a Yao Gui.