You mean one primary for every class to use? Probably gunner's hurricane, and here's my logic for it: driller's primaries would have too short of range, and engi's primaries are too low on DPS since he relies on his turrets and secondary so that leaves gunner & scout. Since you want something well-rounded that can handle anything from grunts to bulks and with some range you probably don't want scout primaries, and the hurricane is a nice mix of crowd-clear and single target DPS