I disagree because I think influence means the capability to affect, manipulate or shape events, development, policy, doctrine etc. and Hitler totally failed to do this. Thankfully he was little more than the tragic end point of the distasteful European experimentation with fascism. Let's face it, his artwork sucked arse too.
It's hard to name one person because many have been influential in sequence. In addition to the others here mentioned, how about: Alan Turing, Marie Curie, Albert Einstein, Alexander Fleming, Richard Trevithick, Justin Bieber.
Depends on how you mean affect, manipulate, and shape events. Do you mean directly or indirectly?
Because if its indirectly, then its without question gavrilo princip. Yes, I change my vote from Hitler.
The number of lives lost as a result of the events set in motion by Princip are incalculable. While some may, and probably will, argue that WWI would have happened without him, the fact remains that WWI DID happen as a direct result of his actions. We can trace so much back to WWI; in part even the Great Depression was caused by that war. The communist revolution in Russia was sparked by the side effects of WWI. Possibly, he is even responsible for the cold war. Hitler probably would have remained anonymous were it not for Princip, WWII might have been averted, or at least the world might have been spared the horrors of the Holocaust.
The effects of his actions are so far reaching, and so tangled, that we'll probably never have an accurate guess of what he cost the world.