Update: More classic insults can be found here

"He has all the virtues I dislike and none of the vices I admire.""A modest little person, with much to be modest about.""I have never killed a man, but I have read many obituaries with great pleasure.""He has never been known to use a word that might send a reader to the dictionary." William Faulkner (about Ernest Hemingway)"Poor Faulkner. Does he really think big emotions come from big words?" Ernest Hemingway (about William Faulkner)"Thank you for sending me a copy of your book; I'll waste no time reading it.""He can compress the most words into the smallest idea of any man I know.""I've had a perfectly wonderful evening. But this wasn't it.""I didn't attend the funeral, but I sent a nice letter saying I approved of it.""He has no enemies, but is intensely disliked by his friends.""I am enclosing two tickets to the first night of my new play, bring a friend... if you have one." George Bernard Shaw to Winston Churchill"Cannot possibly attend first night, will attend second...if there is one." Winston Churchill , in response"I feel so miserable without you, it's almost like having you here.""He is a self-made man and worships his creator.""I've just learned about his illness. Let's hope it's nothing trivial.""He is not only dull himself, he is the cause of dullness in others.""He is simply a shiver looking for a spine to run up.""He had delusions of adequacy.""There's nothing wrong with you that reincarnation won't cure.""He has the attention span of a lightning bolt.""They never open their mouths without subtracting from the sum of human knowledge.""He inherited some good instincts from his Quaker forebears, but by diligent hard work, he overcame them." James Reston (about Richard Nixon)"In order to avoid being called a flirt, she always yielded easily.""He loves nature in spite of what it did to him.""Why do you sit there looking like an envelope without any address on it?""His mother should have thrown him away and kept the stork.""Some cause happiness wherever they go; others, whenever they go.""He uses statistics as a drunken man uses lamp-posts...for support rather than illumination." Andrew Lang (1844-1912)"He has Van Gogh's ear for music."