I think this question is secretly saying "which gun is most like the acr?" :P
Well i guess the colt commando and famas would have to be in that category. Obviously not the same but they both are fully automatic and take maybe 4 to 6 rounds to kill someone. So thats accuracy over power. But of course theres the other assaults like enfield galil and aug. I dont really know how those handle because i havent seen much gameplay of them. And than theres the assaults that pack a punch. Like the ak47 m16 and classified g11 rifles. Those are gona be the strongest assualts damage wise.
I dont think theres gona be a "strongest" weapon. Its all just personal preference. Accuracy or power? What ever your playstyle is.