I prefer bolt action rifles to be honest, due to the fact that I can headshot about 80% of the time. But if I must use an assault rifle I pick the SCAR Assault Rifle from Crysis. It had power, accuracy, all the mods I could dream of, and an ability most guns in most games lack. The ability to change firing modes. I almost always keep it in single shot, firstly because SCAR ammo is rare, and secondly because at that point it just acts like a close range sniper rifle, if I can't get any distance on the opponent.