Does machine gun include Subs? If so the HK MP5A4 if you mean support weapon then the FN MAG without a doubt. However the Bren in .303 is one of the most fun weapons ive ever had the privalidge to use.
It's either a MG42 in 7.92X57mm, M249 SAW in 5.56X45mm, M16 Shrike Combo in 5.56X45mm, M134 Minigun in 7.62X51mm, or my all time favorite Gatling Gun M1865 in .45-70 Government.