-Great topic-. But count me as another who does not see how Jeff Lynne/Todd Rundgren could ever be considered under-rated, especially given Lynne's high profile with the Traveling Wilburys/ or any of the solo efforts he did for the members (i.e. Roy Orbison. Tom Petty, George Harrison)
That being said, he is not one of my favorites because despite his talent, I can tell it's a Jeff Lynne production in less then two bars into anything he does, starting with the Move. I think of a truly great producer as one who can get the most out of the artist's talent, the performance of a lifetime, as opposed to putting their stamp all over it.
George Martin is the best example. The Beatles, Jeff Beck/Blow by Blow are all I needed to hear. His contributions are significant, but all you hear is the artist. He's hardly under-rated.
John Hammond Sr. is anther great choice, but again hardly under-rated. I'm appreciating his "method" of being able to recognize talent and let it be, doing "little" more in most cases than provide insight/encouragement in a direction with a mind boggling track record (Bennie Goodman >> SRV). He did whiff a bit as a producer (imHo!!) on Aretha though as it took Jerry Wexler to put her in Muscle Shoals to realize her magic.
One of my all time favorites is Sam Phillips. He had a relentless drive to settle for nothing less than "different" as the "best" realization of the artists talent however crude and rudimentary it might be. His discoveries hardly need mentioning but I'm not sure he's as widely recognized as a producer. But still, not under-rated.
Al Kooper is another interesting guy who's got his fingers all over some seminal moments in rock history. As a producer he can claim the first releases for Blood Sweat and Tears and Lynyrd Skynyrd.
My candidate might be Pete Anderson and his work with Dwight Yoakum. Some of the best examples of perfect arrangements and sounds to enable an artist's full potential. I find there is a very noticeable difference since they parted ways. I've asked him twice now about his approach and it's that he "thinks like a musician first" and "works with great musicians". Oh, so that's all it takes? .....
A related thread might be -the- single record which best captures the combination of an artist/producer. Something like Grace Jones/Trevor Horn. Or Nirvana/Butch Vig.
Great topic though....