Most people are influenced by many people and things I'm sure but this thread asks for the most influential person on one's style. So I wouldn't interpret single person answers given here as meaning the poster has only one influence.
I answered "My dad". But I also greatly admire Fred Astaire, The Prince of Wales, The Duke of Windsor, Richard Gere (in American Gigolo ), and so many others. But my dad was the one who dressed me when I was a kid, guided me on what was acceptable for which occasions and taught me to tie a tie. He was the first man who I paid attention to. I thought he always looked good and was proud of the way he presented himself. So it is natural I think that his early influence on me is probably the most influential. Though he is no longer alive I still rely on his lessons to guide me in what I wear.