I use the m4a1 with fmj or silencer and it us about a 5 or 6 shot kill without extra shots taken. I have 61 nukes wit it scavenger stoppin power commando (all pro)
personally i like the M16 but it is burst fire which can be good or bad depending on what you think. i would use the ACR or M4. the M4 takes about 3 or4 body shots to kill someone, ACR 4 or 5 but they are both good guns
Scar-H and ACR. Scar because it has a ton of damage, pretty good at all ranges, and solid iron sights so yuo can use something more useful like FMJ for silencers. Only down side to Scar is it's small clip which can easily be fixed by extended mags or stopping power. ACR because it has like no recoil and does a pretty decent amount of damage with stopping power. Also has pretty solid iron sights.
According to some websites, some of my heavily experienced friends, and the CoD main website(I believe) The Tar with Stopping power is the most powerful assault rifle. If you are talking about other guns such as SMG then I prefer the UMP or SOMETIMES the Mini-uzi(if you like to run around and spray for fun) best Sniper: Intervention Best LMG: Aug bar(forgot how to spell lol) but if you are looking for another good assault rifle you should try the Scar(has less ammo tho) or the ACR(best accuracy of all assault rifles but bad without stopping power)