There are so many classic rock songs to get stuck in your head that a number of them simply won't be able to fit, of which there are still so many that it's hard to complete a definitive top ten list of them. Being underrated varies based on a few things, since it doesn't necessarily equate to being totally unknown. A band like Led Zeppelin or Queen can have plenty of big hits and still have songs that relatively few of their fans know or pay attention to.
Maybe it's one of those tracks that people have heard but can't name by title. There's also the factor of how well this composition is treated by those who know this track exists. Such as the band itself: did they not release this as a single, despite the fact that it could have and should have sold really well?