Yes' Tales from Topographic Oceans would be my choice for musical masterpiece. I know no one has probably ever heard it or ever will, the sales sucked, but it has all the elements of music I admire. It's experimental, high concept, incredibly complicated harmony- I love everything about it. How can I resist the most talented musicians of my time putting the Bible to music? Geez, those guys are ostentatious assholes!
Well maybe a few of us have heard of it Madam Z. and it's a great choice. I had the album but lost it along with most of my others in the flood. Those who liked T.F.T.O. might also have liked the album "Anderson Bruford Wakeman Howe", which I think also fits your description of "incredibly complicated harmony."
I think all time favs generally should have certain qualities: a uniform or cohesive theme combined with generally excellent music throughout. So a number of the ones already cited by others above (like Sgt. Pepper) certainly qualify.
Someone mentioned ELO but no title. Maybe they were thinking of the album "Eldorado" or maybe "A New World Record"...both gems.
I couldn't pick a single one. So I'll just toss in a few more of my favs. (See if you're old enuf to remember some of these...):
"MCMXD a.D." - Enigma
"Days of Future Passed" - Moody Blues
"Once Upon a Time" - Donna Summer
"Dive" - Sarah Brightman
"Led Zeppelin" - Led Zeppelin
"Off the Wall" - Michael Jackson (in spite of what we think of him now, it was a great album).
"Meteora" - Linkin Park
"Vapor Trails" - Rush
"Aqualung" - Jethro Tull
"Low Spark of High Heeled Boys" - Traffic (remember that diagonal cut album jacket? Don't you miss album covers? CD's not the same...a lost art.)
"Myths and Legends of King Arthur..." - Rick Wakeman
"Disreali Gears" - Cream
"Blind Faith" - Blind Faith
"Osibisa" - Osibisa ("Criss-cross rhythms that explode with 'appiness.")