My personal favorite is Ma Duece. The Browning M2HB (and its other variants).
Absolute king of the hill and closing in on a century of service.
Thanks to Uncle Sam, and my MOS I have personal experience with a number of full auto weapons, both operation and repair to direct support and general support levels (basically everything except replacing recievers) was part of my job. Also I had an interest in full auto guns before going into the Army, and have kept the interest since.
The M3, M3A1, Tommygun, M14, M16A1, M60, M73, M73A1, M219, M85 and M2HB. Also got to use the MP5, Sterling, FAL, G3, MG1 (.308 MG42), browning .30 (Canadian). I was shown the M240 shortly before I got out of the service, but other than seeing some impressive tests, don't have hands on experience with that one.
The best of them all is Ma Deuce! And its not just the size. The design is not terribly complex, rugged as all hell, and generally works quite well. You can screw them up, and you can break them, but they rarely do it themselves.
The Browning .30 (1917/1919, etc) is nearly as good, being very close in design. The worst of our GPMGs is the M60. Good gun if you are selling them, and spare parts to the govt, a nightmare to keep fixed in the field. Unlike the Browning designs, they simply aren't intended to last, and sometimes don't last as long as they are intended to.
The M73/73A1/M219 is a total loser, horribly over complicated, and the design changes during the series never fixed the real problems.
The M85 should have been a winner, same .50 cal as the M2, smaller, and lighter gun, but when the M60 tank passed away, so did the M85, while the M2 is on the M1 series tanks today.
You can't realistically point to a single weapon and say it "won WWII", but if you could, my vote would be for Ma Duece. If it flew, drove, crawled, floated or swam and was big enough to physically mount an M2, it got one, or more.
And if it walked, it got the M2HB and tripod. not fun to carry around, but well worth it when needed.
THE PRIMARY arm of our fighter planes and bombers, AA and anti material gun for tanks, trucks, boats, ships subs, jeeps, half tracks, Ma Duece got around.
Our enemies had nothing like it, in terms of versatility or widespread use. 13mm & 12.7mm machine guns in some aircraft, otherwise, usually going to a 20mm cannon when going above regular rifle caliber, but nothing like our .50 cal Browning, which even outperformed/out ranged some 20mm cannon.
The M2 has some drawbacks, compared to more modern designs, but NO modern design has proven enough better (so far) to replace the venerable Browning.
The two most common causes for a Browning designed firearm to stop shooting are, #1, the shooter releases the trigger, or #2, it runs out of ammo.
Ma Duce is also a very accurate machine gun. Scary accurate until the barrel wears. Rifle accurate. Actually too accurate for best "cone of fire / beaten zone" results, so a large tolerance was built into the ammo to compensate for this. AND the M2 is also a selective fire weapon. And, it can be set up to feed from either side. And it is fairly simple to mount optics. And, and, ...
Show me another gun that is that versatile, if you can!