James Fenimore Cooper. Mark Twain wrote (to my mind) an absolutely hilarious piece called "Fenimore Cooper's Literary Offenses," that catalogues the awful inconsistencies in Cooper's writing. You should read it. What you shouldn't read is Cooper himself. Caught up in the Romantic view of native Americans, he filled his novels with noble savages who were just short of Godlike in their abilities in the natural world. The main character of his Leatherstocking series is a white man raised by Indians. This person, Natty Bummpo by name, (and yes, it is just ridiculous), has been given amazing abilities by his adopted culture. What he hasn't got is a lick of consistency. He talks like a Boston brahmin in one sentence and like an ignorant redneck in the next. Cooper had a tin ear. I know there's a Fenimore Cooper society out there, but I won't be joining.
Herman Melville. I confess that I actually like Moby Dick, but that's because I enjoy digressions and description. Read Moby Dick and you could probably run a whaling ship yourself. The trouble with Melville is that he's inconsistent. Sometimes he rises to amazing heights of insight, as he does with Captain Ahab. Other times, he wallows in pathos, writes awkwardly, and forgets what he's doing. I put "Bartleby the Scrivener" on the same plane as Charlotte Temple. Melville matters, though, in who he influences. His picaresque novels inspire a whole generation of writers like, well, Mark Twain, Hart Crane, Stephen Crane, and their ilk.
Harriet Beecher Stowe. Mrs. Stowe's novel Uncle Tom's Cabin, was a 19th Century blockbuster, and on the positive side, it brought to light the appalling conditions that slaves and their families faced. It humanized the slaves by showing whites that slaves had emotions that were just like white emotions. This came as a shock to some people, even in the North. The problem is that, on the negative side, the novel is both polemic and sentimental. Stowe also couldn't sustain the effort, and nothing else she wrote reached the pinnacle (if that's what it was) of Uncle Tom's Cabin. Oh, and she wrote in dialect, and did it better than Fenimore Coooper, although a ten-year-old would've been better than Cooper.