Agreed that AK's win the numbers game; they are the most widely used, because of being cheap, reliable, and simple to make.

For actual quality, and combat effectiveness, the M16 or M14, probably about a toss-up. M16 more versatile, can use it for sniping to 800 yards, all the way down to CQB, and can carry more ammo. But the M14 has more oomph and a longer range.