As some have already posted 'The Gathering' is a good choice. Easily my fav Testament album. It also ranks in my top 25 metal albums of all time. The musicianship is superlative, the production is flawless and, all things considered, as far as American, well known, thrashy, non-Pop metal albums goes this is (IMHO) second only to 'Reign In Blood',
Also worth buying is 'First Strike Still Deadly'. A compilation of older tunes, rerecorded by most of the members of the 'Gathering' players. Very, very good.
I saw Testament open for Priest on their 'Painkiller' tour in the US supporting their 'Souls Of Black' album. Was not impressed but I'll admit I was a bit ignorant as to what was good back then.