IN SEARCH OF THE LOST CHORD

Though the Moody Blues are probably best known (at least in the USA) for their pop-rock songs from the 80's, they were doing some good innovative music in the late 60's / early 70's, often with lots of groundbreaking mellotron colorings.

I'd consider this to be their best album. It definitely contains pop elements, and is a light in intensity compared to most bands, but it contains some powerfully rich melodies and songwriting. Again, the mellotron has a strong presense, as well as tasty vocal harmonies, flute, and various persons in the band taking turns with songwriting and singing. This might take a little while for some people to get into, but once this sinks in, it can be fairly rewarding.