Not really sure what qualifies as "obscure," as it is a relative term... but if you've only heard Springsteen's BTR and later albums, you're missing his best, imo: The Wild, the Innocent, and the E Street Shuffle.
Or, when was the last time you heard John Denver...? How about Rocky Mountain High album...
Not really obscure if you're a Willie Nelson fan, but if you're not familiar, then: Red Headed Stranger or his gorgeous album Stardust...
Simon and Garfunkle: Bookends.
And for you rockers, better than their BTO Greatest Hits album(s): Bachman Turner Overdrive: Not Fragile
T Rex: Electric Warrior