The first 3 to 4 Testament albums are considered thrash classics in my opinion. One of the best thrashers to not feature in the Big Four along with Overkill and Exodus. Obviously not everyone will like them but there is a reason people like these albums, they consider the first 3-4 albums thrash metal classics and so do I. The Legacy and The New Order particularly are amazing albums.
I think "Souls Of Black" suffers production wise and "Practice What You Preach" is not really very good at all to be honest. "Formation..." was a blast if the new one is anything like that I'll be making a purchase real soon.