i love most of the songs that testament made but The legacy would have to be one of my favorites. it's a masterpiece. one of the best songs i've ever heard.
Everything off 'The Legacy' (especially 'Over The Wall' and 'Burnt Offerings'), plus 'The Preacher', 'Practice What You Preach', 'DNR' from the new stuff...that'll do me for now.
Every single song they've written is worth hearing! Listen to the entire The New Order album. Or if you wan't something heavier, listen to the entire The Gathering album
I would say either: "Practice what you Preach title song", "The Preacher", "Malpractice", "Love to Hate", "Face in the Sky", "The Ballad", or "Raging Waters".