I've never gotten why Angus was considered a great guitarist either. Then again, I'm not a really big AC/DC fan. At the risk of calling down the flames, I've got to say that I've always thought Eddie Van Halen was a little overrated, too. Some of his early stuff is inspired, but later on, particularly in the Hagar era, there just doesn't seem to be much passion there. I seem to be the only guitarist of my generation that wasn't inspired by EVH.

Underrated ... there are a lot of guys out there that I think are underrated. Hard to pare it down to one.
Tony Iommi. Certainly not for his lead work or his technical skills, but in his early days, he was, IMO, the greatest of the riff writers.
Billy Gibbons. I love the tone and feel of his earlier, bluesier stuff. I could sit and watch the guy play all day long.
Jeff Waters. Being Canadian and not one of the big thrash acts of the time, I think Waters got lost in the mix by a lot of folks, but his first couple of records were really inspiring for me.