I'm amused to find that people count
Dune,
The Lord of the Rings and
Catch-22 as "Classic" novels.
I think that for any novel to be judged as the best of "all time", it would have to demonstrate that it had appeal to several generations of readers. And the only way to do that is for the novel to be in print and being read for many, many years. At least 50, and more likely 100.
Which is why, if asked for the best "Classic" novel, I'd tend to only consider pre-WWII novels, or perhaps even only pre-20th Century novels.
But which to choose? My choice rather depends on which one I've read most receently
So if I had to choose just one, at the moment I'd go for
Kidnapped by Robert Louis Stevenson.