Who is your favorite under-appreciated guitarist?

That's the problem with a term such as "under-appreciated"; people interpret it differently and it is subjective in a weird way because it is not only about assessing one's own opinion but also about the opinions of other people. It may mean "almost unknown" but on the other hand if you think that Andy Latimer is the number one guitarist in prog then you could legitimately claim he is under-appreciated despite him being quite well appreciated overall... one could also argue that it is entirely different from "unknown" and rather means "better than people think" which is an empty statement as long as somebody is unknown so not many people think anything about this person.

Some guitarists that for me a really among the very best in prog and generally are Adrian Belew, Fred Frith and Peter Wolbrandt (of Kraan). All of these are more or less well known and even well appreciated, but you'd probably find them on few top 10 lists of guitarists and they'd certainly feature on mine.

Lifeson, Rabin and Barre have between them sold well in excess of 100 million albums.

Rabin is also a fabulously successful composer of Hollywood film scores and the single "Lift me up"  which Rabin co wrote with Squire for the album "Union" spent 6 weeks at the number one position on the billboard album rock tracks chart. 

Latimer also clocks up album sales in the multiple millions. 

Justin Hayward has recently been inducted into the Rock and Roll hall of fame.

Jan Akkerman was well appreciated as a guitarist in the 70s and Focus even had commercial singles success completely outside the sphere of prog or fusion. 

Ant Phillips is an oddball who suffers from stagefright which I believe was his primary reason for quitting Genesis - if he had stayed he would undoubtedly have reaped the rewards. I believe he is a very fine 12 string player who has contributed to several albums for others outside of Genesis and on his own - he is perhaps a qualifier. 

Peter Banks may also be a qualifier but I must confess to not knowing any of his work outside of his fairly minor contributions to Yes - there is an article about him on Wikipedia which describes him as "the architect of progressive music" (a bit off the mark perhaps). 

Admittedly I haven't heard of the rest of the guitarists on your list so they may well also be qualifiers but I really don't think that Lifeson et al as mentioned above in this post have a look in on this poll....as successful and famous as they all are.....

In my own (probably irrelevant) opinion the term "underappreciated" equates very well with the term "unknown".   Most of the bands and musicians who are listed as artists here on PA and virtually all of the posters on this forum who are guitarists are completely unknown and equally unappreciated...

Not sure if he's underrated but Donald 'buck dharma' Roeser from BOC....at least I don't see him on many lists.

I'll go with Jean and say Roman Bunka of Embryo. Such a great guitar player, and he plays the oud too! And his first solo album "Dein Kopf ist ein schlafendes Auto" is one of the very best albums of the 80s and among my personal favorites. And no-one but Jean and me ever mentions him, which is an absolute shame. You guys should most definitely listen to the examples Jean gave in her post.

Someone named Eddy Marron which is not a bad call either.