Well, I think Starship Troopers has got eberything else beat, in that it not only ignores alnost completely the plot of the book, but it gets the tone completely wrong, turns hard-science SF (with some necessary imaginary science) into scientific illiteracy, and completely inverts the book’s philosophy, It’s rare that you find everything so completely turned on its head. Good effects, though.
Besides those already mentioned, let me mention that, besides science fiction, spy films are the least like their nominal source material. For some reason, they figure they can scrwe with those as much as sf/fantasy with wild abandom. Examples:
The Spy Who Loved Me – most James Bond movies by this point had little or nothing to do with the books, but in this case they had an excuse – F;leming expressly forbid them using the orignal story. About the only things in common are that there was a character named James Bond, there was a woman who was intimate with him, and a villain had steel teeth (low-cost Eastern Bloc dental work, though, not the stylized lethal choppers that Jaws ported.) Moonraker, which followed this, had precious little to do with the book, either, but that wasn’t Fleming’s doing. They just wanted to cash in on the success of “Star Wars”.
The Osterman Weekend – Robert Ludlum gets the same treatment. It was the warmup for all the Bourne movies, in that regard.
Ice Station Zebra – well, it’s got a submarine, and the North Pole, but Alister MacLean got screwed out of his story in this example. They can’t all be “The Guns of Navarone”