Personally, I think that it's a tie between Who's Next and The Who Sell Out.
Who's Next is basically one long "rock and roll is great bombast" recording, Sell Out was one of the very first "concept albums."
Sell Out features some really great songs, like Armenia, City In the Sky (featuring Townshend's backwards recorded guitar solo.) Other greats are I Can See For Miles, Rael and Mary-Anne With the Shaky Hands, plus the hilarious entries of Odorono and Tattoo.
The CD re-release features extras not found on the original recording as well as a handy pamphlet explaining the 1960s days of swinging London and pirate radio.