THe greatest prog disco song is by Mike Oldfield Platinum Suite (side A of the Platinum album especially part 3 and 4 of the suite)
Herman Hermits - I have no idea what the song is about as the lyrics doesn't register until I read the lyric sheet but I found it melodic enough and I am not surprise that Carole king wrote the song. Regarding Herman Hermit, if you have songwriters of the calibre of Graham Gouldman (of 10cc fame) and Carole King writing the songs, it really doesn't matter how dorky they are and how non-rockist they are.
Randy Newman is currently ranked number 185 of all time in my list. So I doubt I would qualify for that group
The 10cc song I posted is no where near 10cc greatest song. I only listen to one of their albums (Bloody Tourist is the last decent album) from the last batch of what's spinning. If I was introducing someone to 10cc I would probably pick Rubber Bullets. Sort of The Beach Boys cross with Abbey Road medley without sounding like a medley.