One and only one belt-fed machine gun? Very simple in the extreme. MG-42. In the original 7.92x57mm caliber. Why? Simple. I don't fly planes. I don't drive tanks. I have no combat experience. What I do know about these things has been gained by reading, and by talking with church members, etc., who have been in combat. From what I gather, a machine gun has got to be supremely reliable, portable, chambered for a serious caliber and capable of being effective at actual engagement ranges. The MG-42 is superior in all these respects.
If the frame of reference turns to air combat, vehicle mounted usage, etc., then I would prefer a .50 caliber M-2.