Honestly, I dont know i dont think it was underrated I think it was well known and respected. To this day it is still my favorite Cannonball album too. But still I am not sure about being underrated but it is a classic jazz album in my opinion. I may even put it in my top 5.
When put in to the context of the larger canon of his work, Miles Davis' "Bag's Groove" is, I believe, a truly underrated album. The line up of players on the record is exceptional and the tunes are all now considered standards. I love Miles and Milt Jackson playing together, I think their sounds compliment each other really well (also check out the Miles Davis/Milt Jackson Quintet/Sextet recordings).
Andrew I agree with you 100% percent. Bags Groove as well as Miles Davis and the Modern Jazz Giants are two underrated albums. 1. Because of everything he did later on for Columbia, it kind of overshadowed his great prestige outings. 2. I feel because this was never an actual band (meaning they never were in concert together) its not looked at in the same light, which is totally unfair.
Since everyone names "Brilliant Corners" I'll name the one other Monk album I think is as great--"Misterioso" and I definitely mean the 1958 Riverside album not the Columbia catch-all of the name that came out later. For some reason, that 1958 quartet with Johnny Griffin, Ahmad-Abdul Malik and Roy Haynes is enjoyed without being celebrated that much, but both this and "Thelonious in Action" were recorded the same night at the Five Spot and are both great. It was a wonderful Monk group.