My penis. Oh yes, I went there. Having been at an all boys boarding school from grade 7-12, it's hard to rub off the desire to make silly comments most people over 18 don't find funny. But anyway, I would say the best assault rifle of all time would be the AK-47. It's really the universal gun. When you think of a non-pistol gun, you think of the AK-47. Apparently they go for about $50 on the black market in the Middle East.
If I went into battle though, I wouldn't want the Ross Rifle. For those of you outside Canada, or who didn't have Alberta's premium Xtreme education system, the Ross Rifle was TERRIBLE. Snipers loved it, but when charging across no-man's land with a long and heavy rifle, regular soldiers HATED it. It all starts with the British, who's Lee-Enfield rifle was the best in the world at the time (around WWI). The government didn't like the idea of using British weapons, so enlisted the services of some guy who's last name was Ross. They wanted to make a Canadian rifle that was shorter, less expensive, and lighter than the Lee-Enfield. Instead, they got the Ross rifle, which was longer, more expensive, and heavier than the Lee-Enfield. It was the gun of choice for snipers, but everyone else hated it. Soldiers would pick up the Lee-Enfields of fallen British soldiers. Eventually, this was disallowed, and anyone caught with a Lee-Enfield would have some kind of punishment, I don't know what. That was a fairly silly chapter in Canadian history. The government also used cardboard boots to to combat trech foot. Ok they didn't even think of trenchfoot, so they weren't quite that silly, but that doesn't reduce the stupidity by very much.