I don't know if you could call them underrated, but one guy I like listening to when I'm chill is Al Green.
Otherwise I have a lot of Beatles albums, Rolling Stones albums, I listen to some Bob Dylan stuff, the Who, a variety of different oldies that aren't heard on the radio anymore, Led Zeppelin, Pink Floyd, the Police, the list keeps going on.
It's nice to find a rare gem once in awhile and not hear it a million times on the radio or a commercial IMO.