Personally I feel that "classics" are books that enlarge ones point of view or somehow teaches one something. Perhaps it's a book that is written well or is extremely useful to understand the time period. "Classics" are in a sense books that help a person a lot, that give them more wisdom or intelligence.
Alexandra said that
Alexandra Steele wrote:there are plenty of so-called classics that I could happily have lived without ever reading
Yes, but even those books are necessary to understand what was going on in the mind of the author and similar people of the time period. Of course, there are books that should have been considered classics but are not. However, that's a different problem.
In the end, classics are books that people should definitely read, whether it's a great book or completely horrible. Even if something is horrible you should still learn about it, intelligence isn't just knowing the good in the world.