Although nominating "overrated novels" (here and here) sounds like great iconoclastic fun, I can't quite get over my gut reaction: "How do we define 'overrated'?" I've had grumpy reactions to several canonical novels, for example, but considering the topic from the POV of a literary historian, my grumpiness on aesthetic grounds is totally irrelevant. If the novel has influenced generations of successors, then my aesthetic objections are neither here nor there: I still have to teach the book (if it's in my field, anyway), whether or not I want to throw it into the nearest recycling bin. Then again, from the POV of literary history, it's quite possible to "overrate" a novel's importance (the extent to which it attacted imitators, was genre-changing or -originating, etc.) by pointing to its aesthetic superiority. It's the great irony of critical evaluation. (It's also the great challenge of developing a survey course.)
So, to dwell in my usual field of religious fiction, John Henry Newman's Loss and Gain is, I think, an overrated conversion novel. Its quality as a novel is leaps and bounds beyond that of just about every other nineteenth-century conversion novel--not perfect, to be sure (Newman never knows when he has exhausted a joke), but still, a twenty-first century Victorianist can make it through Loss and Gain without suffering doubts about his or her academic vocation, eating an entire bag of Lindor balls (preferably the dark chocolate variety), or deciding to write about Star Trek fan fiction instead. And, of course, Loss and Gain is by Cardinal Newman. (Never underestimate the selling point of a famous name.) The thing is...the more Victorian Catholic fiction I read, the less significant Loss and Gain looks. It went through many editions, but it's not the book other Catholic novelists chose to imitate. In other words, the best novel turned out to be a one-off.
By the same token, if by "overrated" we mean "novels that have had an influence that they shouldn't," my critical self is willing to go to town, but my literary-historical self says, once again, "in the end, that's not particularly relevant." Like the Denver Bibliophile, I was deeply unamused by my slog through The Lord of the Rings--great world-building, prose like wading through the Slough of Despond--but nobody interested in the history of fantasy would dare ignore it.1
1 A student once threatened me with a bow-wielding Aragorn figurine after I dared to hint that, perhaps, the novels were not all that they could have been.