It is the Bessemer process that makes modern society possible. Without a way to make large quantities of high quality steel quickly most things many of the things that are critical in our civilization would not be possible or at least not possible on such scales.
There is some debate in whether or not some came up with close to the same time, but there is little evidence of this. It could be that had Bessemer not invented his process the entire world would be different.
The Wright Brothers
This is another option as without real planes it would limit the scope of war from the last century... For example, another name who's a possibility may not have mattered without the plane.
Without him our view on the universe would be significantly different. And like the Wright brothers, even though many people were working on something similar, if it weren't for it being developed at that moment the world would be different and there is no telling when, if ever such advances were made. The idea of the atom has been around for 3000 years after all. It is the fact that it could be utilized when it was that may have saved the human race from a later catastrophe.
There are others that could possibly have huge impacts that we're just not as certain about. Bessemer is only really amazing in retrospect. Richard Feynman is probably someone who is very important, Alcubierre might be, Donald Trump might be, but we can't tell as of yet, positive or negative. All presidents from the last 30 or so years will only be marked as important if something really Earth shattering happens that can be tracked to their idiocy. Any positive outcome will bare out that they weren't important at all. There are plenty of people I can't tell whether they are incredibly important due to lack of knowledge. Kim Jong-il and Putin may be very important.
There is a temptation to say Hitler, Stalin, JFK, Truman, etc were important, but many of them when you know about their positions, where they come from, and the general situation of the time, they are not that important at all.
Lincoln died 2 years too early for this but he might have qualified, not because really anything he did, but because what happened due to him existing. Kill him at 15 and no Civil War and possibly a more socially cohesive US with no need for the Civil Rights movement... and tons of people would be alive. Franz Ferdinand is another one that his existence is more the linchpin than anything he did, had you killed him earlier it is possible that WWI and WWII would not have happened, or at least not at that time. Though, really with the malarkey that were all the alliances and treaties that was bound to happen eventually.
All that being said I place my bets on Bessemer, The Wright Brothers, or Einstein being the most important humans in the last 150 years to have existed that we currently can quantify.