The Beatles' Sgt Pepper album. There has been so much retrospective criticism of this thing, that I think these days it would actually qualify as underrated!
Yes, there's lots of trippy, whimsical, celebratory fun to be had, but the album always keeps one foot firmly planted in the real, far-from-utopian world: the narrator of "Getting Better" remorsefully admitting to being a wife beater, the eerie ambiguity among the already exquisite melancholy of "She's Leaving Home," and, of course, the masterful final track, "A Day In the Life," where the average citizen stays willfully blind to the weird chaos surrounding them.
Finally, let's not forget that one the Beatles' best non-album double A-sides, "Strawberry Fields Forever"/"Penny Lane" was a product of the Pepper album sessions!
I'd even daresay that the Beatles should have broken up after Sgt Pepper. The best songs from the later Beatles albums would have shown up on their solo recordings.
One thing I will say about the Beatles is that whoever is responsible for the standardization of their discography is an unsung hero. With pretty every other British invasion band, having to worry about the British/American/Canadian/etc. versions of their albums is really annoying when you are trying to listen to or collect everything.