I am going to cast my vote for the M2. Not a gun many of us will ever own or even fire but...
This thing has soldiered on from war to war to war.
It has been mounted on aircraft, ships, boats, vehicles...even robots I think. It has been mounted in ones, twos, fours, and I think there were some WWII aircraft (the type escapes me) that had 16 of them all pointed forward on one point...it was a light bomber that they modified to be a heavy coastal attack plane.
The thing has been used to suppress enemy troops, defend against aircraft from the air, defend against aircraft from the ground, served as an dogfight weapon (probably the only thing it isn't still doing today), used against ships, used against vehicles, and used against mines.
It was actually designed by John Browning himself.
There are, as far as I know, no plans or even thoughts of replacing it in the 21st century.
The Ma Duece is easily the formost contender for continuous active military service for 200 years and will hit the 100 year mark without breaking a sweat.
M2 .50 Caliber Heavy Machine gun...all time best gun ever.