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.
I wish this was more talked about, when people mention classic sci-fi. I'm an avid SF reader, particularly older stuff, and it could almost be a drinking game of how few pages it'll take before you find an offensively outdated reference, no matter how great the book. But every time I've picked up a Heinlein, hoping to find more positive points in classic stuff, I'm left just...feeling ooky. An easy example being the lesser known Friday, with the "happy" part of the extremely unrealistic female protagonist's journey: marrying one of her gang-rapists. I haven't been able to make a dent in my stack of Heinlein's since that nonsense. Too many other great and interesting authors that weren't horrid shitbags.