Rush Limbaugh, when I was in high school. It wasn't so much that he made me change, but that he made me realize I was conservative. When you're young, you don't really know anything about the political parties, but you get some vague idea that one is good and the others aren't (and the one that's good is usually liberal.)

Mind you, the Overton window in Canada has shifted so far left that even the People's Party is too far left for my tastes.