John Lennon: Plastic Ono Band (1970)
Lennon's solo debut was perhaps the most focused and soul-baring, standing up alongside All Things and Ram as a monumental achievement that left no doubt these artists had plenty more to give after the break-up of the Beatles. (And, as with All Things, it would prove to be a solo peak difficult for Lennon to replicate, though I think Imagine is underrated.)
As much a shared therapy session as a musical work, the album has Lennon ruminating on his deceased mother (the stunning opening track "Mother" and closing coda "My Mummy's Dead"), religion and faith ("God"), love ("Love"), isolation ("Isolation"), and so on. As with the Beatles' close-out statement Let It Be (recorded before Abbey Road but released later), Phil Spector helped produce the album; but whereas Spector's symphonic touches arguably muddled parts of Let It Be (I'm partial to the alternate mixes), here the sound is largely left in its original stripped-down state, mostly just Lennon at his piano or with his guitar, with some unobtrusive backing. This leaves the music to stand or fall on its own, Lennon's intense and focused lyrics standing front and center. And while the far more studio-accentuated and radio-friently follow-up Imagine may sound more like a Beatles record, Plastic Ono Band is the more riveting document of Lennon's psyche.
I probably listen to Imagine and some later albums a little more frequently (or the hits collections, given Lennon's ongoing ability to conjure up some stellar singles even amidst his lesser work), as the intensity of this one makes it less amenable to mere background play; but Plastic Ono Band is Lennon at his best, showing the artist without McCartney or commercial pressures pushing him into a more polished place.
Here's a video for "Mother":