Why is Wuthering Heights considered one of the best novels of all time?

Probably because her characters are not perfect, they don't always make the right choices and things don't always go as planed. She writes about the real human nature and all our flaws. Her novel was considered shocking and wrong when she wrote it, and that might have helped to its popularity

I know that this is an old question, but I wanted to answer it anyways.
For me there are so many reasons for its lasting popularity.
I think part of it is the oddness. As the last person said, these are not perfect people. They can be weak, selfish, bratty, sadistic, controlling, and a million other things. And yet some how we find ourselves liking them, या at the very least interested in them.
For me this is best demonstrated द्वारा the main couple. Catherine can be bratty and selfish and (spoiler alert) she dies halfway though the book. Heathcliff himself is very flawed and does some monstrous things. Somehow, though, the प्यार and passion that these two have for eachother makes us want them to be together.
That is another thing I think makes it last. We dont get the typical happy ending for those two. They dont end up together in life. However, there are certainly indications that they ended up together when they died, which makes this a प्यार that even death couldnt conquer.
Things are also left open interpretation. Is that really Catherine that they see at the window and on the moores? Is she a ghost? Is she in hell? Is she in purgatory? Its up to आप to decide.
Finally, I think there are lairs to the story that makes it last. I have read it probably 3 या 4 times, and each time I see something in the characters and story that I didnt see before.