The AK-47 for a few reasons.
Old M16s were shitty in construction and function, and accuracy, but those have all been majorly addressed since the 70s, so those don't make as much difference.
I can strip an AK-47 and rebuild it blindfolded in less than 2 minutes because it is so simple. Sometimes Simple Is Good.... just ask SIG. It's not the most accurate, but it's passable, and anyone intelligent never rock and rolls. Also, the durability of the gun is legendary. I saw an AK run over by a tank, and the guy used a wrench as a hammer to beat the barrel straight, and it still worked, less accurately, but still functioned. Also, the thing actually gets more accurate when its a little dirty, and almost never jams, unless you have shoddy ammo. There's something about having wood for a stock that just feels right to me. Never warmed up to synthetics.
The M16 has developed into an admirable gun, but I have never been the fan of the 5.56 NATO. I prefer the 7.62x51 (similar to a Win .308, but slightly hotter) which I use for my Palma rifle, or a 30-06. 5.56 is a varmint round, like the .22 hornet magnum. The 7.62x51 and 30-06 are varmint rounds, but for the ten-toed variety.