Some people consider Fleetwood Mac's "Rumours" to be a concept album, the concept being the break-up of established relationships and the start of new ones.
Jethro Tull ~ "War Child"
It was a re-working of the earlier album "Passion Play", and was intended to be the soundtrack to a film. The basic concept is that a young ballet dancer, who is pregnant, is killed in an accident, and then her soul is fought over by Heaven and Hell.
The expanded remastered edition makes slightly more sense, because of the inclusion of extra songs.
I have 5 other absolute favourite concept albums, but I've no idea what the concepts were! Mike Oldfield's :~
"Tubular Bells" ; "Hergest Ridge" ; "Ommadawn" ;
"Incantations" ; "Platinum"
And I also love these :~
Jethro Tull ~ "Songs from the Wood"
Pink Floyd ~ "The Dark Side of the Moon"
Pink Floyd ~ "Wish You Were Here"
The Beatles ~ "Sgt. Pepper's Lonely Hearts Club Band"
I know, I know, but I've never been good with singular favourites!