beatles white album, controversy caused at the time because of it means that it has to be mentioned in this sort of discussion, but i'm going to have a look before i commit myself to just one favourite...good question
Led Zep Four, because it doesn't mention the name of the band anywhere on the sleeve, and still shifted shedloads of copies. One of my faves is another Floyd album - Meddle - with the close-up of the ear - says it all really.
Not an album, but New Order's "Blue Monday" 12" sleeve was a seminal classic that cost so much to produce Factory Records lost money on every one that was sold.
Possibly anything by hipgnosis would have to be high on the list....The Wall and Wish You Were Here as well are rightly considered classics....modern albums really have been blighted badly with the much smaller CD format as opposed to the 12" LP covers of old......But if your twisting my arm then I'll go for Momentry Lapse of Reason by the Abdabs!
It's got to be Led Zeppelin III, when it was originally issued on vinyl it had a revolving wheel inside the gatefold sleeve, look at the quality of the artwork. The CD has just been reissued in a replica sleeve.