Who is the most underrated prog rock guitarist?

I agree with the comment about speed - it's about what you say, not how fast you say it. Also agree about Hackett but I'm not sure that I'd call him under-rated as he is generally recognised as one of the top prog guitarists.

My number 1 underrated guitarist would be Nick Barrett of Pendragon.

Again, I wouldn't say Lifeson is underrated, certainly not in the prog world.

The problem with 'rating' Gilmour is in prog alone, Latimer, Hackett, Rothery can manage that soulful shtick as well as Gilmour AND also have more range than him, especially Hackett.  And if we look at Gilmour's 'direct' competition, which is really blues-rock guitarists (because that's the style he plays most often), then it's very tough to make a case for him being better than the likes of Duane Allman, Gary Moore, Stevie Ray Vaughan, etc (or bluesy hard rockers like Blackmore - ok, not JUST blues - and Angus Young). I am not sure I would rate him over Martin Barre for that matter.  I mean, if the argument is Gilmour fits the music like a glove, so does Barre. 

Speaking of speed, how about speed AND complexity AND flavour? Mitch Watkins has it all in this solo:

I think the guitar is a very rangey instrument, capable of sounding extremely aggressive and flamboyant in ways not many other instruments do (except the saxophone which was, not surprisingly, the quintessential jazz instrument for many years).  So...why should slow and sentimental playing alone be elevated to the top of the pack? I think that Mitch Watkins solo is also soulful in a completely different way; it is invigorating and puts a big smile on your face rather than making you cry.  Is that really such a bad thing, must all art always be gloomy?  Mind, in saying all this, I AM a huge Floyd fan.

I think Gilmour is a better composer - both of tracks and of guitar parts - than a player. And a lot of the credit he gets for his playing is really intended for how good he is at writing parts.