Some important works, groups of works and composers from the medieval period 1100-1400:
Troubadours, Trouveres (Bernart de Ventadorn, Thibaut de Navarre, Colin Muset et.c.)
Minnesänger (Walter von der Vogelweide, Neidhart von Reuental et.c.)
Notre-Dame school (Leonin, Perotin)
Cantigas de Santa Maria (Alphonso el Sabio)
Estampies
Carmina Burana
Missa Tournai
Guillaume de Machault
Llibre Vermell
Italian Ars nova (Landini)
Ars subtilior (Anthonello de Caserta, Johannes Ciconia)
Some importatant musicians and ensembles in casual order which have recorded medieval music and whose recordings can be safely recommended:
Studio der frühen Musik, München (Teldec and EMI):
Extensive discographie including almost all kinds of medieval music.
Ensemble für frühe Musik, Augsburg (Christophorus):
Clemencic Consort (Harmonia Mundi , Oehms and Arte Nova Classics)
Mala Punica (Arcana, Erato and Harmonia Mundi)
Ferrara Ensemble (Arcana, Harmonia Mundi)
Micrologous (Op 111)
Emmanuel Bonnardot and his ensembles Alla Francesca (Op 111, Virgin, ZigZag) and Obsidienne (Calliope)
La Reverdie (Arcana)
Ensemble Gilles Binchois (Cantus and Virgin)
New London Consort / PhilipPickett (L´Oiseau Lyre and Linn)
The Dufay Collective (Avie, Chandos and Harmonia Mundi)
Ensemble Unicorn and Oni Wytars (Naxos)
Paul Hillier and his ensembles Hilliard Ensemble (ECM, EMI and Hyperion) and Theatre of voices (Harmonia Mundi)
Gothic Voices (Hyperion)
Sequentia (German Harmonia Mundi)
Ensemble Organum / Marcel Peres (Harmonia Mundi)
The early Music Consort of London / David Munrow (EMI and Archive and Decca)
Musica Reservata (Philips)
Huelgas Ensemble (Pavane and Sony)
Anonymus Four (Harmonia Mundi)
Add to this a list of groups with only a few but important recordings:
Martin Best Mediaeval Ensemble (Nimbus),
New York´s ensemble for Early Music (Lyrichord),
Theatrum Instrumentorum (Arts),
Ensemble Anonymus (Analekta),
Tonus Peregrinus (Naxos),
Boston Camerata / Joel Cohen (Erato)
Modo Antiquo (Brilliant Classics)
Diabolus in Musica (Studio SM)
Orlando Consort (Archiv and Harmonia Mundi)
I have heard almost everything these groups have made of recordings of medieval music, and I find virtually nothing, which doesn´t pay in one way or the other. It is just to get started.
A fine starter might be the Harmonia Mundi 6CD release "Les tres riches heures du Moyen Age, containing recordings by among others: Ensemble Organum, Clemencic Consort and Anonymous Four.
Also the Early Music Consort of London´s three CDset "Music of the Gothic area" on Archiv,
the Machaut Mass by Clemencic Consort on Arte Nova,
the Llibre Vermell and Cantigas de Santa Maria CDs by Theatrum Instrumentorum on Arts,
the Messe de Tournai by Ensemble Organum on Harmonia Mundi,
the "Ars subtilis Ytalica" by Mala Punica on Arcana,
the Machault Motets by Hilliard Ensemble on ECM,
the recordings of secular Machault works by Gilles Binchois Ensemble on Cantus,
the Estampie collections by the Dufay Collective on Chandos and Avie,
the Estampie collections by the New York´s Ensemble on Lyrichord (2 CDs)
the "Beaute Parfaite" by Alla Francesca on Op 111 (containing Ars subtilior-works)
and Paul Hilliers solo CD "French Troubadour songs" on Harmonia Mundi.
As I said: Just to get started.