As I've said all along, I'm not a fan of the concept of 'underrated' or 'underappreciated' art. It seems to me that any work of art gets exactly as much appreciation as it deserves. To argue otherwise is to argue that there is some absolute standard of artistic value to which some people, mysteriously, have access, while others do not. To claim that something is 'underrated' is merely to say that our own tastes differ from most other people's.
I do think, however, that some music albums are just so good that the 'excellent' songs overshadow the merely 'very good' songs. The whole album sets such a high standard that the listener becomes spoiled, and forgets just how good the 'second-rate' tracks are compared to what's on most other albums. Only in time, when the shock of the new wears off, do listeners begin to appreciate just how good these 'second-rate' tracks are. So our appreciation of these songs grows with time.
I would say that "Holiday in the Sun" by the Sex Pistols falls into this category. It's their fourth and forgotten single, after "Anarchy in the UK," "God Save the Queen," and "Pretty Vacant," and it rocks. A great opening track for a great album.
Another neglected gem from a classic punk-rock album would be "Forward to Death" by the Dead Kennedys, from Fresh Fruit for Rotting Vegetables
LP. It too could have been a fourth single, after "California Ueber Alles," "Holiday in Cambodia," and "Kill the Poor." It's not as innovative as those other three, or even "Chemical Warfare". But it's just so angry and hostile, so relentless and hard-driving, that it epitomizes hardcore punk in a single song.