Robert Heinlein. His works were all over the place, and it would be a mistake to assume that he believed in something just because he wrote it in a fiction novel, so on that front I think he gets a fair amount of undeserved heat. He was pushing the limits of progressivism for his time, tossing out and seemingly defending everything on all sides from fascism to anarchy to direct democracy.

If we discount his fiction, though, since it can be hard to tell from that what he actually believed, then he still falls pretty short by modern standards. A homophobe, almost certainly racist, and although he was practically a feminist by the standards of his time I would have to admit that he's pretty misogynistic by our standards.