Hitler was incredibly influential...
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.