Jon Anderson backed by Chris Squire and Steve Howe, as others have already pointed out, is probably the best combination of voices among the big prog bands.
Gentle Giant (Shulmans plus Minnear) - again, plenty of posters have gone for them, with good reason.
Zappa's band on Sheik Yerbouti also achieved a very rich and deep vocal sound, as did the slightly later line up with Ike Willis and Ray White.
Koenjihyakkei's current line up has some incredible vocal harmonies, something that Yoshida Tatsuya also incorporated into Ruins Symphonica.
Beating all of them, however, is the mighty Magma. The current line up with Stella Vander, Isabelle Feuillebois, Antoine Paganotti and Himiko Paganotti is absolutely amazing.
Edited by Syzygy - May 29 2007 at 07:02