While I don't care for them personally (believe me, I've tried to like them), I'd vote for the AK47 because of it's famed reliaiblity, durability, inexpensive production, and it's by far the most favored battle weapon of the globe for going on 50 years or so. Fires the perfect war caliber. Big enough to stop men reliably, penetrate light armor or cover, not be upset by deflecting off small branches in jungle fighting, and small enough to carry a lot of ammo comfortably.
That is pretty impressive. Used heavily by the Russians, Chinese, Vietnamese, all or the middle east, south American nations, African nations, etc.