That's a hard one.
I change my mind all the time about my favourite novel, but that isn't necessarily the best novel of all time?
My current favourite
Oryx and Crake by Margaret Attwood
My vote for best novel of all time (nominated with a large dose of uncertainty)
Gulliver's Travels by Jonathan Swift
because it is a fantastic read and has stood the test of time. Although it is a political satire of its own time it remains remarkably relevant today.
Of course, it's arguable that Gulliver's Travels is not actually a novel, pre-dating as it does the modern novel. However, it is equally arguable that the novel has had its day and that the post-modern "novels" we all read now are not strictly novels at all. I'm not sure that I'd agree with these arguments but the question at the top of the thread is vague about what a novel is.