Underrated to me has always meant "doesn't get the popular attention or praise he/he deserves." By that criterion, if you compare the praise/acclaim that the "big three" of English guitarists (Clapton, Beck and Page) from Mick's era are rated with what the four of them actually do -- then Taylor is way underrated.
But if you want no doubt completely underrated, consider Steve Morse, David Hidalgo of Los Lobos and Joe Bonamassa. Steve Morse gets plenty of critical acclaim, but for a guitarist of his ability he is way underrated by the public at large. Of all these, the guy that I think most captures the spirit of "I had no idea he was this good" is Hidalgo. Consider that Clapton has invited him to both Crossroads festivals now. Bonamassa is just not very well known -- but those who do know him recognize how insanely good he is. And Morse works with a band that is widely believed to be past its prime (Deep Purple) -- which is probably true, but doesn't take away from the fact this guy is as good as it gets in rock.