Hmm. The classic 1931 Frankenstein isn’t even remotely faithful to the book. More recent versions come closer (The underappreciated Victor Frankenstein AKA Terror of Frankenstein from 1975, Kenneth Branaugh’s version from 1994 (wjichj is splashier, but less faithful) and the 2004 miniseries (closest to the novel, but the Monster is downright good-looking) are better.
BladeRunner departs pretty significantly from Philip K. Dick’s Do Androids Dream of Electric Sheep?
The Invisible Man from Universal with Claude Rains follows Wells in many ways, but updates to 1933, adds personal details and plot complications not in the book, and screenwriter Philip Wylie added things from his own Invisible Man story. But other adaptations stratyed even farther from the book.
The 1984 version of 1984 was awesome and incredibly faithful (right down to being filmed in London in 1984, and with John Hurt looking disturbingly like George Orwell)
War of the Worlds – both film versions took considerable liberties with Wells’; novel. Much as I love George Pal and Steven Spielberg, I was disappointed.
20,000 Leagues Under the Sea – the Disney version has a lot going for it, and is better than just about any other adaptation (to see how wrong they can go, have a look at the part-silent 1929 film [The Mysterious Island, which is really an badaptation of 20,000 Leagues. And not a good one.). I love it, but it still differs significantrly from Verne, especially when the Disney film suggests that Nemo’s Nautilus is powered by a nuclear reactor like that (then) contemporary Nautilus. And Nemo’s base at “Vulkania” is destroyed by a mushroom cloud-producing device.
Things to Come isn’t really based on Wells’ The Shape of Things to Come, which isn’t a novel. One might as well try to base a film on Aldous Huxley’s Brave New World Revisited (which isn’t a sequel to Brave New World). In fact, the film didn’t really have much to base it on at all, which was part of the problem. It has awesome visuals, and stirring pro-science optimism, but it’s really kinda dumb. When Stanley Kubrick finished watching it, at Arthur C. Clarke’s suggestion, he reportedly told Clarke never to recommend a movie again.