My choice:
The M-16 (or a RR AR)!
No, not because 'this is AR15.com', but...
How else can you get a full-length rifle, submachinegun, pistol (in the vein of the H&K MP5K or the 'pistol UZI'), and even a LMG (24" barrell and bipod, with either mag or belt feed) all off one gun, and in your choice of caliber? If only there was a 'shotgun upper', it would be 'the universal gun'...
As for other comments:
1) A 12.7mm gun is essentially a 50cal...
2) That MP44 reminds me of an AK... Oh well, the Russians have allways had a nack for copying other peoples weapons...
3) IIRC, there is really no 'production' man-portable GE Minigun... There were a few that were CUSTOM MADE for use in various movies (Predator, Terminator 2), but they were never mass produced (mainly due to the tremendous amount of power needed to operate them. The guy in Predator had a 220VAC electric line running from the gun down through his fatigues, and out his pantleg. Not very practical for military use). And both were 5.56mm 'shrunk versions'. The actual minigun is, of course, 7.62mm (getting the term 'minigun' from being a 'miniature' version of the original Vulcan 20mm cannon)...
The power issue is also why downed helo aircrews in Somolia and Afganhistan could not use the door-mount miniguns after going down: without the engines running to turn the onboard generators, the miniguns couldn't shoot...