Are we talking about best performance, best impact on battles or best impact on gun design.

If we talk about greatest impact the AK-47 has to be near the top, it was the first lightweight, reliable, and mass produced assault rifle (STG was heavier, and the Germans couldn't mass produce it)

I think the Ma Deuce (M-2 Browning .50 cal) is one of the better guns, look we still use it.

The Colt 1911 and Luger should both be on the famous guns list because they started the whole automatic pistol thing.

Kar should be on the famous guns list because all the other bolt actions were others' answers to the Kar.

If we talk about impact on battles I would have to say Garand and PPSh SMG. They were mass produced and deadly in the hands of the right allied soldier.