Your favorite 'concept' album

I'm not a concept album fan, however there are a few I've gone down the rabbit hole with. My personal fave, is '10,000 Years' by The Honeydogs. The backstory & lyrics are so vivid, and futuristic, it never fails to reveal something new to me. Even the bandleader said at times he's not sure what it's about.

Fascinating info & interview in the url. Check it out! 

Any others? 
A lot of good ones have been mentioned. I want to add Kilroy Was Here by Styx. 
As audite mentions tons of good stuff.

My own is Aftermath.  Can anyone figure out the concept?
Triumvirat - Sparticus, Rick Wakeman - Journey to the Center of the Earth, Genesis - The Lamb Lies Down on Broadway
Will skip the obvious one like 'Tubular bells' and post Tangerine Dream's


On somewhat lighter note

Brian Eno 'Apollo'