NFL - John Elway. I dunno why, but I loved the Broncos when I first decided to actually pay attention to football. Mr. C was not pleased.

NBA - Michael Jordan. I didn't watch basketball much at all but the Bulls and Jordan were always, always, always talked about in school and watching them play during his peak was really fun. And I fucking loved Space Jam and it made me a fan of his forever.

MLB - Mike Piazza. I was the only Mets fan in school and it was miserable, especially during the Subway Series. I remember being REALLY mad when Clemens threw his bat at him.