To me it's Andy (Andrew) Beyer of the Washington Post and Daily Racing Form. His speed figures are used all over North America and led to a lot of the new ones. The good thing about Beyer is he tells you what he is betting.
Mike Watchmaker is also really good. He sets Morning Lines. http://www.drf.com/news/watchmaker-watch-weekly-divisional-ratings
There's an apparently crazy British guy named John McCrick. He dresses really wildly but does pick winners. He is really fun to watch.