Good, or great!
Preferably 70s (and late 60s)… If YOU think it’s prog, please paste the YouTube link… Duets are cool, too, and the blending of certain voices can be quite great. I keep running into them, and many are not well known.
I tend to love soothing, voices in a high register, with piano, violins, slower-tempo, but of course there are exceptions. I notice many of my favorites might be considered “classical” (piano), like the song “Face of Yesterday” by Illusion.
Here’s one I love… I’ll post more if the thread gets going.