The only real qualification I consider for an album to be "double" is that it comes with two CD's, not LP's. You could technically have two LP's and have the album simply continue onto the other record, but "double albums" usually have full albums separate from each other, either in tonality or lyrical effect. This also means that I do not include "Greatest Hits" albums or collections because that denotes a collection that was most likely not chosen by the artist themselves.
Here are my few favorite double albums:
The Beatles - The Beatles (aka "The White Album")
The Beatles - "Ob-La-Di, Ob-La-Da"
Bob Dylan - Blonde on Blonde
Bob Dylan - "I Want You"
Led Zeppelin - Physical Graffiti
Led Zeppelin had the early 1970's. Hands down. They took Rock 'N Roll and Blues to places people probably only imagined in their darkest desires. After five hugely successful albums, Physical Graffiti landed and only continued their streak. Every song is in the land of Zeppelin, if a bit more casual that usual, and we're along for the ride cruising down Route 66. It's a fine dose of rock that's unprecedented.
Led Zeppelin - "Trampled Under Foot"
Pink Floyd - The Wall
Pink Floyd - "Comfortably Numb"
Smashing Pumpkins - Mellon Collie and the Infinite Sadness
Smashing Pumpkins - "Muzzle"