well if you're talking the best prog rock band overall then, here ya go...

and the most under-rated prog rock band has gotta be these dudes
Wouldnt call Pink.Floyd progressive rock any more than i would call Radiohead prog rock....just great rock bands that push the sonic envelope. But "they" classify them prog rock because they didnt sound like Zeppelin.
King Crimson is definitley "prog rock" ....which is actually jazz/rock fusion which was pioneered by Jeff Beck and Jan Hammer on "blow by blow" and "wired". I would not say they are underrated. Maybe by Joe Neckbone who thinks "Freebird" is awesome, but not by people who know music. Robert Fripp is retarded.