Using the Walker and Sepiks from Devil's Lair as an example, for Heavy Weapons the Machine Gun is the obvious choice. Rate of fire wins the day here, presuming you have the ammo.
Primary/secondary weapons you would have to go Scout Rifle. Numerous people were able to solo the Walker in the Lair by finding a spot to avoid the cannons and pick at it's legs. This usually required a fair amount of range, making the SR the clear choice, again with it's rate of fire winning out over the Sniper.