here are five of my current favorites:

Orlando Consort -- John Dunstaple: Musician to the Plantagenets
Tallis Scholars -- William Cornysh: Stabat Mater
Gothic Voices -- The Mirror of Narcissus - Secular Songs by Guillaume de Machaut
Venance Fortunat Ensemble -- Fulbert de Chartres: Cantor of the Year 1000
Ensemble Organum -- The Chant of the Templars - the Holy Sepulchre of Jerusalem in the 12th Century

and i also have to mention Benjamin Bagley and the late Barbara Thornton and their ensemble Sequentia, for, among things, two wonderful traversals of the more-or-less complete works of Hildegarde von Bingen.