It is very surprising that after all these responses I don't recall seeing anyone mention Jon Eardley? Jon is best known for his recordings with Gerry Mulligan. He also recorded with Zoot Sims, Phil Woods, and has a Prestige session under his own name.
However, the guy I believe to be most underrated is Bill Hardman. I recall going to a club in New York city some years ago and seeing a marvelous quintet with Bill hardman, Junior Cook, Walter Bishop,Jr., Paul Brown, and Leroy Williams. Hardman truly knocked me out that evening.
Hardman is on a number of recordings with Jackie Mclean, with Art Blakey and the Jazz Messengers,and recordings under his own name on Savoy and Muse.
Other trumpet players I consider especially underrated are:
Tommy Turrentine
Lonnie Hillyer
Ray Nance
Carmell Jones
Joe Gordon
Thad Jones
Peter F