Definatly a sniper rifle. With a steady hand, you can rack up a lot of kills depending on the distance, and if it was silenced or not. It conserves ammunition, takes less bullets to kill, and a whole lot more powerful.
I would use a 50. cal semi auto sniper rifle with an ironsight attatced to it so i can shoot them at close range (and mabey a knife on the end of the barrel if they come "too close").
sorry for speling mistakes im Dutch.
well if it's call of duty then mostly machine gun but if it's certain maps on halo reach then sniper and for battlefield 3 then once again depending on the map i may choose one or the other