I voted 'old'.
I really like the first five, including the Ritual. Technical haunting thrash metal. Real good stuff. I don't know which one would be my favourite album, maybe Practise What You Preach. I never really got into the post-Skolnick Testament, maybe because Skolnick always had a good sense of melody and there was also a lot of intricate playing. I guess I found the later stuff a bit too dumbed down for me. Not to mention they started sounding too much like a lot of other bands. Just got lost in the shuffle.
I love Formation of Damnation though. It has good parts of old and new Testament. And a killer production to boot.