Classical music covers a huge path of time and space and also happens to be at a very low popularity. Anonymous 4 almost qualifies as a supergroup in the genre.
Four women singing mostly medieval sacred music. They've been doing it wonderfully for about fifteen years (sadly they just announced that they will stop touring next year) - so wonderfully that they've sold more than a million recordings .. a feat almost unheard of in classical music.
If they are performing within a day's drive it is worth the effort to go to a concert. Their disography has about a dozen entries. I recommend:
La bele Marie
1000: A Mass for the End of Time
Legends of St Nicholas
A Star in the East
On Yoolis Night
An English Ladymass
I'm sure their other CDs are great - I just don't own them. I will spare you a detailed review other than saying their music comes as close to perfection as any singing you will hear. Etherial comes to mind.
Their site has concert dates, news and a few real audio samples. Buy a CD or two and see if you can avoid buying a half dozen more.