The best player ever? I'd have to do some research into who played a year or two of c before moving up, but Keith Crowder certainly comes to mind. I've had my favourites over the years, like Aaron Mulcaster, or, right now, Tyler Scott, but it's certainly a rare thing to see a Junior c player make the NHL.
The thing you have to factor in though, is that there's no Junior A south of Toronto, so everything gets shifted down a letter artificially. From all the time that I've spent watching both leagues, I have no trouble saying that the GLJHL and the EOJHL are pretty close in terms of quality of play. Considering that, everyone who had ever played a year of Junior B at 15 before moving up in the rest of the country would have played Junior C in SWO. For example, Jimmy Howard played for Westport (B) at 15, and Tie Domi played for Belle River (C) at 15.
Of the top of my head though, I can't think of anyone who played his whole Junior career in C and made the NHL.