While Hitler had more of an immediate impact on the events of the past century.
There are just too many events and social trends that can be traced to Hitler and/or World War II -- the cold war, the jet plane, wholesale genocide, the use of the media to further his ideology (print, film, radio -- even early experiments in broadcast television), the development of the U.N. and international law (as a post-war reaction to Hitler), and also the carefully orchestrated photo op/propaganda event (i.e. the 1936 Olympics). These things were all unique to this century, and all came out of or were directly linked to Hitler's Germany. I in no way mean to say that we ought to appreciate these things as good, but the impact and influence they had on the era is enormous.
But if your criteria is grander, then Einstein wins out. Modern science and really even our worldview as a planet were heavily influenced by Einstein's theories.
When historians in the distant future look back on this era, Einstein will be seen as the major -- really overwhelming -- influence.
Henry Ford is a good choice, too, but there were a lot of inventors working along the same lines. The Model T did change the world, but my sneaking suspicion is that someone else would have hit on the same idea of an affordable car for the masses eventually (and, who knows? even done a better job).
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