In my opinion a classic album has to be something that no matter your mood you can put it on and it can take you somewhere else or make you feel something else. Deja is my favourite album of all time but it took me a few listens to really appreciate it, so it doesn't have to be something that instantly grabs you it can easily be something that grows on you. Everyone's idea of classic is different, I'm not a Metallica fan but I wouldn't ever disagree if someone said one of their records is a classic in the genre, same applies to some major pop artists just because I don't get it doesn't mean it isn't a classic in someone's eyes, it's all subjective.