Charles Mingus - Adagio ma non troppo
Holy shit. This composition would be a noteworthy achievement even if it were actually a composition. However, it's not technically a composition, since the entire thing is based around a song Mingus improvised at the piano, an instrument he didn't even usually play. Let My Children Hear Music (if the whole thing maintains the quality of the first two tracks, which I have no doubt it will) and The Black Saint and the Sinner Lady firmly establish that Mingus was legions ahead of almost any other musician of the 20th century, possibly including Miles Davis. Just spellbinding.