Gormenghast: "impenetrable dated prose," "phallocentric" (I have no time for this), and one-third posthumous mess.
Little, Big: You have to be high to like this book. Turgid, indulgent, meandering, full of fey females viewed through smeary vaseline.
Winter's Tale: Misogynist magical realist wish-fulfillment wankage by a Luddite jerk.
The Sword of Shannara: Widely hailed as beginning of the multi-volume-epic-fantasy extruded-Tolkien-product phase of fantasy publication. I've never even tried to read this book, but I did get partway through Magic Kingdom For Sale--SOLD!, also by Brooks, and had to stop because if it was in earnest it made me want to vomit and if it was parody it was poorly executed. Worse than most fanfic by native English speakers.
Chronicles of Thomas Covenant: Maybe the first really successful dark fantasy series, but reads like something written by a zitty, embittered teenage boy studying for the PSAT. There was a blog a few years back where two girls read Lord Foul's Bane a page at a time and discussed it. Can't find it, but it was hilarious.
The Lion, the Witch, & the Wardrobe: Transparently didactic crap. Made it easier for fantasy to be dismissed as kids' stuff.
ETA: The problem with lists like this is that people make rookie mistakes, like mistaking "greatest of all time" for "thing I enjoyed the most when I was 13" or "greatest novel" for "greatest collection of short stories." Magic for Beginners is very good, but it is not a novel, and has not been out long enough for the perspective sufficient to determine whether it is one of the greatest of all time.