I'm going to skip the overrated bit, cos the subjectivity involved tends towards meanness. There's shedloads of stuff I don't like or don't get, some obviously decent, some obviously crap.
Underrated -
I'll never understand why the Wildhearts aren't bigger than they are. Album after album of great tunes, and a phenomenal live act at their best.
Ditto Jellyfish. Spilt milk is a work of art, it should have been everywhere at the time, but it was largely ignored. As soon as spring comes around I'm drawn to listening to it, and it's as perfect now to me as it was on first listening.
Trouble. The end of the 80s was a pretty stale time for the more traditional rock/metal scene IMO, and I bought the 1990 self titled album on spec, knowing nothing about it other than from the band shot on the front it might be my thing.... It was that day I discovered that it was possible to shoehorn groove into those kind of songs. 30 Years later and its still a favourite. I bought Soundgarden's Louder than Love and Alice in Chains' Facelift around the same time, and though more rooted in the past it still stood proudly alongside them for me.