This is really tough. I reckon it could change every day.

Nuclear Assault - New Song
Exodus - A Lesson in Violence
Possessed - Evil Warriors

Actually, this could change every hour. However, there is something about New Song that would pretty much make it a keeper. I think it was the first thrash song I heard outside of the Big Four - in fact, maybe before any Anthrax. Oh and Lone Justice is probably my fave Thrax song. Brilliant.