lmg
even with an LMG, if there are five people standing in front of you, and you just start spraying at all of them, you're probably not going to get lucky enough to kill more than one before they can react, if you even manage to kill one at all. if you use ADS, you're almost definitely going to kill one, and then you can try to do that to the next, if they still haven't reacted
I've never had much luck doing either while using a weapon without a suppressor, though