RAM was under rated when it was released although uncle albert/admiral halsey was a hit and I suspect the album sold pretty well and these days it is generally rated in the top part of Paul's solo output.
Here are some I can think of:
Kevin Ayers - Yes We Have No Mananas
Ayers deliberately avoided stardom by refusing to tour very much. But this album is full of good songs, I think pif it as an unknown classic. Most Ayers fans wouldn't put this as number one though.
Soft Machine - Softs
Karl Jenkins had become the principle composer, he later became the most popular living classical composer. But this album is full of beautiful music, that's really exciting. I always felt it should have been much better known. In a similar vein Gong's Gazeuse.
Van Dyke Parks - Jump!
Best known for working with Brian Wilson on SMiLE, but this album shows he could write clever songs and do amazing arrangements. Also the other album he did with Wilson "Orange Crate Art" is a really under appreciated gem.
Moon Safari - Blomljud
Swedish proggy band with great multipart harmonies. Very much under the radar but create great songs.
Rich Goodhart - Shaman Mirror Medicine Tree
An online friend of mine. But this album shows a lifetime devoted to making his own way and an ability to capture amazing timbres on tape.
Andrew Baker - Various
I just recently stumbled across this guy who has been composing since the early 70s and has a fully developed artistic vision expressed in a number of media, film, writing & music. Generally they're performed on composing software. But he's my obscure recent find, I find his work fascinating.
I've probably taken the obscurity thing a bit further than intended, but it kind of lined up with what I'd been thinking about.