Looking at all the lists, I just see people picking the shotguns that they like and or know. I don't see a why.
I would say:
1-The Winchester 1897- First successful pump shotgun Started the whole trend and why they made the 870, 500, or Nova in the first place. I know there was the 1893 winchester pump but with the 2 5/8" chambers it was not as popular and there was the Marlin pump gun but again it was a) not a popular and it was made after the 93.
2-The Browning Auto 5- First successful Auto loading shotgun leading to the 1100, 1187, M2, Super Black Eagle and any other gewiz auto you can think of.
3-The group as a whole of LC Smith, Baker, Parker, and Fox Sterling for showing that an American company can put out a side by side every bit as good as any of the the English firms at prices most shooters could afford.
4- The Ruger Red Label- First gun to come out as a 20ga and later made into a 12. Ok the Browning super posed that started the popularity of the over and under. I just like 20ga and like that the Red label (my uncle calls it a Pink lable cause of the 20 ga) and was scaled up to 12 rather than scalled down to 20 as most other models are.
5- NEF, Iver Johnson, and H&R single shot top breaks- With out these in expensive "Starter" guns not too many would have gotten a start hunting as kids. With this we now have shotgun designed as youth models.