To a first approximation I think Brian Jonestown Massacre, although Dig! and their Boardwalk Empire theme music helped thrust them into the mainstream.

To a better approximation I think Kritical Audio is a seriously under-appreciated glitch artist, as is Seven Ark, although it should be noted that Seven Ark is the early work of Justin De Nobrega, AKA DJ HiTek, the music composer behind South Africa's version of Insane Clown Posse, Die Antwoord, which is a tragedy beyond all proportion.

Ultimately I think its hard to point out talented but chronically underappreciated artists, since in this day and age, internet media like Youtube and Bandcamp have helped bring even the most obscure music to the masses, and there is enough people jonesing for good music, whatever the taste, that any talented artist given time will accumulate the appropriate audience. This subreddit is evidence enough of that.

The one undisputed musical genius that I hardly ever see posted in any of the many muscial subreddits I subscribe to is the late Pete Namlook, creator of the FAX label, and his collaborators Tetsu Inoue and Dr. Atmo. Now that is some serious electronic space music on par with the best music composers that have ever existed, in my opinion.

but that's just my opinion.