I'll give you two of my own to start -- and I'll probably cause a flame war just for my opinions on T.H. White. I'm a medievalist at heart, and a junkie for the fantasy genre, who eagerly picked up a copy of The Once and Future King one summer as a break from studying Old and Middle English. I thought it was a crime I hadn't read it, since I do own multiple copies of the Morte d'Arthur and promised my professor I would read Sir Gawain and the Green Knight in the North Midlands dialect someday.
I expected to be blown away, to sneer at Disney's milquetoast adaptation, and put it there alongside my umpteen copies of Chaucer. Instead, I could barely get past the cat boiling, the threats against Questing Beasts and hedgehogs, and the blatant misogyny. (Guinevere is a basket case because she can't have kids!) Disney's The Sword and the Stone may not be my favorite film, it may not even rank among the greats of animation, but it's better than The Once and Future King -- if only because it lacks cat torture and misogyny. (It does, however, lack Robin Hood, which is the highest point of White's novel for me.)
My other pick would have to be Doctor Zhivago. This is one of my favorite films, and I dutifully read the book. It's a good book (like the film, it's far more about the Stalin era than it is about the 1917 Revolution), it's poetic, and it's romantic. Some of the most cinematic parts are even left out of David Lean's classic. But Lean, at least, understood that the story's heart was the war-ravaged romance between Yuri and Lara. It's their relationship that symbolizes the life, love, and beauty that the regime is suppressing. I know Boris Pasternak knew it too, and yet betrayed his own characters with an unforgivable ending. If you cherish your memories of Yuri and Lara, never read it. For years, I was convinced that something was lost in the translation, and that there was something deeply Russian I was missing ... only to have my native Russian professor suddenly blurt out the same thing in history class one day.
So, if your a White or Pasternak devotee, you can argue the point now -- but I would much rather hear some of your own suggestions. Debate away in the comments ... and be brave about stating your picks!