I guess "A Night At The Opera" could be ranked as a "classic album"?
I've heard some people try to make that argument, but is it REALLY loved like Sgt Peppers, Pet Sounds, Abbey Road, Tubular Bells, Rumours or even BiA? I don't think it is.
Of course you could count in sale figures from all these as well to compare their quality. I think there is room enough for different views. Mentioning BIA, it is a classic album for sure but most of the time it is not so much mentioned for its musical quality but for the CD momentum thing. Sales are six times higher than A Night At The Opera though so you have a point.
LE
Absolutely, it's all about opinions and there's no hard and fast rule as to what makes a "classic" album.
I've been absorbing all this stuff in books, magazines, online and even (gasp) in person interactions for 40 odd years and I've just never got the feeling that A Night At The Opera is loved and "broke through" to a wider audience like, say, Nevermind by Nirvana.
Queen's Greatest Hits is another story which begs the further question... can a compilation be a classic album?!