My two favorite parts from his Vespro della Beata Vergine, 1610. Monteverdi is fast becoming one of my all-time favorite composers, and this is why:
From the wiki: “The text Duo Seraphim (“Two angels were calling one to the other…”) begins as a duet. When the text (which melds lines from Isaiah and the Gospel of John) mentions the Trinity, a third tenor joins. All three sing in unison at the words, “these three are one.””
I own this version.