A combination. In his Preface to The Great Gatsby, Michael J. Bruccoli writes,
Literature has staying power, but it is subject to metamorphosis. Every reader's response to a work of fiction is determined by his or her presuppositional biases, beliefs, experience and knowledge.
Fiction can provide much more to the cultivated reader although it can provide entertainment, escape, and insight to even the rudimentary.
"Should the main purpose of fiction be to entertain, to educate, to provoke thought, or a combination of these?" is a question whose answer defines the meaning of "a classic." Classic literature provides all levels and thus is timeless as it can appeal to the child, the ingenuous, the sophisticated, the jaded, the experienced, the poorly educated, the erudite.