I agree with Izzy Stradlin.
I'm not sure where I sit with Thom Yorke. Like Hetfield he is highly regarded as the band's frontman, but he may be underrated as a rhythm guitarist. I find it hard to decide because I don't that the guitar makes Radiohead good. They are great for other reasons.
Hetfield and Scott Ian are two of the rhythm guitarists I definitely don't see as a unsung. Hammet garnered a lot of attention, but Hetfield was the primary songwriter so he got what he deserved imo.
Personally, I think Gary Rossington was underrated. I guess he was slide guitar, but whatever.
Dicky Betts in the Allman Brothers was underrated as well, at least until Duane Allman died.