JUST KIDDING!
I don't know about underated guitarists, but I will say that John Paul Jones of Led Zeppelin is an underated bass guitarist. He does some great basswork and his mandolin is great on battle of evermore. He is kind of underated, because Robert, Jimi, and the other John get a ton of well deserved attention, but JPJ needs more.
Also Joe Satriani, and when I say that, I mean he is underated in that you will never hear him on mainstream radio. Fortunately though, he gets the respect he deserves from the guitar world.