AK-47, the most popular and widely used assault rifle in the world. It is well known for its substantial reliability even under harsh conditions and very low production costs compared to contemporary Western weapons.
During my military training program, I was impressed by its sturdiness and easy maintenance, where you can dissemble and reassemble the rifle blind-folded, which is extremely helpful at guerrilla warfare. And its simple structure helps it to operate normally in extremely dusty conditions where rival M-14 and M-16 would fail.
Sometimes I even think that without AK-47, Viet Cong perhaps would not have won the Vietnam War.