At least in my schooling, Upton Sinclair was held up as the very model of a muckraker, and he was certainly anything but (2) above. Perhaps sensationalist on occasion, but strongly principled.
Roosevelt was applying the term to journalists at publications like McClure's. There's a great book about this period by Doris Kearns Goodwin (The Bully Pulpit) that I can't recommend highly enough.