So a quick revisit to this thread, and an observation...
I'm gonna throw out there that there's a big difference b/w underrated and unknown. As being discussed now.
I saw Scott Henderson listed very early on, and I'd venture to say those that know of him, know of his incredible talents.
There are a bunch more like this, already mentioned on here.
Oh, and as far as Alex Lifeson...in the 90's on Rolling Stone Magazine's top 100...just plain silly!!!!
THAT'S UNDERRATED!!!
And Zappa, killer guitar player, but I think many just know of his humous lyrics, not his guitar prowess...which was incredible!!!