I voted for The Legacy. I think all of Testament's albums are good except for Demonic. The song Demonic Refusal is ok but I cant handle a whole album like that. The Gathering was really good and First Strike Is Deadly was great. Another personal fav of mine is Practice What You Preach.
I feel guilty picking just one since ALL of Testament's albums are killer. I pick The Gathering because it sums up elements of all their previous albums.
I voted for 'Practice What You Preach' simply for its significance in MY metal music history. But based on my current tastes, I'd go with 'The Gathering.'