"The New Order" to me is definitely the best. " The Legacy" is a good pure thrash album, but I think TNO just has more to it musically. TNO has a crapload of awesome songs that are played live all the time ( Eerie, Title Track, Trial, Into the Pit, Disciples, Preacher).
Believe it or not, I actually think Souls of Black but be their second best. It's a truly underrated album...sure , the production isn't that great, but songs like Face in the Sky, Malpractice, and Seven Days of May should be payed more attention to and played live.
I'm having a hard time decide which my second favorite, that or the Legacy, but who cares, it's the overall best, so like I said, The New Order.