Testament is one of my all time favorite bands, period! With that said, I'll give you a run down of what I think is essential for you.
1.
The New Order - In my opinion this is Testament's answer to "Master of Puppets".....essential? VERY!
2.
Practice What You Preach - This album got me into the band and is excellent throughout. A great place to start in my opinion.
3.
The Gathering - This is modern Testament without Skolnick, but don't let that fool you. This is a brutal album, but it still has that Testament groove.
By the way, get the Live In London DVD...sheer excellence!
~Brian~