I would have to say Metallica is the best thrash metal band. Especially with the release of Death Magnetic and their return to their older work showing the true domination of Metallica. Plus, they are being taken into consideration for the Rock and Roll Hall of Fame.
Metallica is definately one of the best. minus that whole St. Anger thing which idk if you can really call thrash... or even call it good (but who cares about that xD, nobody liked it anyways :P ) other than that, they basically defined the genre.