The prior post mentions some of my favorite composers-- and performers!, but I'm afraid those composers are all renaissance era, a little later than what you were asking about. Tallis is probably borderline, time-wise, but he was Byrd's teacher, so you probably wouldn't find his style sounded as medieval as the usual representatives.
Machaut is maybe the most well-known, at least he gets talked about and performed/recorded a lot. Well, "a lot" relative to the genre, I guess. Also Landini, Perotin, Leonin. For performers, if you could find some old David Munrow albums, he recorded everything, including lots of vocal music. He was really, really good. He took his own life when he was pretty young-- tragic. Anonymous 4, a group of women in New York (one from the UK) recorded gobs of medieval music, wonderfully sung. I'm not sure they are still together, but there are many, many recordings, each with an interesting and often obscure theme. And Jordi Savaal, who I thought was more of a baroque specialist (gamba), seems to have directed some medieval recordings, and anything he does would be outstanding. His might be mostly instrumental, not sure. Have fun!
Topic: Medieval Composers