I don't know how underrated these are, but they're very good records:
Taft Jordan: Mood Indigo
Zoot Sims: Quartets - dodgy sound on a few cuts, but very good music
Phil Woods with Red Garland: Sugan - one that Chuck turned me on to
Getz, Zoot, Cohn, Eager, and Brew: The Brothers
Elmo Hope Trio: Meditations
Charles McPherson with Carmell Jones and Barry Harris: Bebop Revisited - a good bop date - w. Nelson Boyd on bass - years after the fact
Tiny Grimes with J.C. Higginbotham: Callin' the Blues
two volumes of the Wardell Gray Memorial
Finally, the musicians may not be underrated, but I don't see these recordings mentioned very often:
Jackie McLean Quintet: Lights Out! - with some very fine Elmo Hope
Benny Golson: Groovin' with Golson - He and Curtis Fuller made a fine team.
Art Farmer: Farmer's Market
Gene Ammons: Nice an' Cool
A special tip of the cap to many of the Swingville recordings. My listening and jazz listening in general would be much, much poorer if Prestige hadn't recorded so many swing musicians. No other company did this to anywhere near the extent that Prestige did.