1) The most influential in opening the West was the Transcontinental Railroad. The Transcontiental Railroad opened up the West because it gave access to millions of citizens the opportunity to move to western states much quicker and cheaper than previous methods of transportation (such as horse and carriage). Along with this, the railroads allowed for easier trading making it so that Westerners could easily sell their goods to national markets.
4) One could argue that the Bessemer process had the greatest impact on the growth of the US as an industrialized society. The Bessemer Process allowed for the mass production of steel at a much chaper cost than ever before. This resulted in steel being used to build skyscrapers, railroads, etc. None of our current day large cities (New York, Chicago as examples) would have been able to grow if it were not for the Bessemer Process.
5) John D. Rockefeller was arguably the most influential businessman of the late 19th and early 20th centuries. John D. Rockefeller created Standard Oil Company. This company, at its peak, would control 90% of oil made in the US. This made John D Rockefeller the richest man in American history.