Totally, totally agree about Angus Young being overrated. I'd also have to say Eric Clapton and Keith Richards. *ducks flaming toilet paper* Don't get me wrong, I like AC/DC and Clapton and the Stones as much as the next guy, I just don't think they're all that amazing as guitarists. Especially Keith Richards.

As for underrated, I'd say Nick Drake is the big one. He was quite an excellent fingerpicker, and (as far as I know) one of the first to use really outrageous tunings.