If you want the truly underrated then you ought to include the bands almost no one ever heard of that still produce outstanding music. Yesterday's Children is the first one that comes to mind, along with Lucifer's Friend, Orang-U-tan, Pink Fairies and a plethora of other that slip what used to be my mind. If you have time to kill get on Youtube and listen to any of these, then just start randomly listening to all the similar bands the are offered.
One band which did have some commercial success but was definitely underrated was Wishbone Ash. FWIW, Tull is probably my favorite band but I don't know if I'd say they were underrated, TYA certainly was, as was FZ during his lifetime, can't agree that his buddy the good Captain was. A lot of worthy names on this thread.