I don't mean to beat a dead horse, but:
Winner. Although he may have been more appreciated if they could have played his music. He had the audacity to record music that woudl nto pass censors.
But, couple his genius compositional skills with his insane guitar talent and yes, he was WAY underrated.
I read an interview with Zappa once, where he talked about that very subject, obviously a sore one with him. He talked about doing an interview for a radio station, and asked why they were interviewing him when they wouldn't play his music. He pulled one of his albums out of their rack (Hot Rats, which has only a short vocal on ONE song), and asked when the last time they played anything off of that album. The DJ's response was about how the censors wouldn't let them, and Frank's reply was, "The censors have a problem with instrumentals?"
Musician? Yes. Underrated guitarist? I don't know about that. He hired Stunt Guitarists for a reason. He was a great guitar player but I don't think he's underrated in that regard.
Stunt guitarists??? I would hardly call Lowell George, Steve Vai, Adrian Belew, and Warren Cucarello "stunt guitarists!
9 out of 10 has never heard of them? I'd bet that majorities of people have heard of them, excluding the teen set. Admittedly, Zappa's fading because he's dead and wasn't an actor as well.
I might agree Waits is underrated because he's one of my favorites, but this is where these under- and over-appreciated threads fall apart for ,e.
Most people who listen to rock have heard OF Zappa, but few have actually HEARD HIM.
Now, as for underrated musicians, IMO one of the most underrated talents was Jerry Garcia. He was an excellent guitarist, and one of the best pickers ever. To be able to record with the likes of Merle Saunders, regularly, is a testament to that fact. The problem is that A) he played with the Dead, who critics loved to dismiss and fans looked at as a hippie cult band, and B) bluegrass is under appreciated as a musical form.