How to identify a classic album
It’s not easy to objectify when an album qualifies as classic. However, some of the main ingredients are clear. Ideally it should have outstanding songs with surprising or catchy melodies and thoughtful lyrics, skillful or even groundbreaking playing, excellent vocals and preferably a state of the art production. It also should bring in something new that wasn’t there yet; it has to be ahead of its time. The album has to be influential in any way, e.g. because it initiates a whole new subgenre. And it helps when rock journalists agree the album is a masterpiece.
It’s hard to predict how influential an album will be. So classic albums generally are released some time ago. Although commercial success isn’t a necessary condition in itself, it generally helps an album to be influential.