Here are the ones I suggest. I will be going off general quality and not accuracy, but will make some notes if I know the book well enough.
The Invisible Man
1933 version with Claude Raines. follows the basic premise of the book (although not exact) and on it's own is one of the best horror films of the era. Incredible special effects for the time.
HP Lovecraft's stories
Murky waters here, and you will hear many opinions. I suggest watching Re-animator if you can stomach the gore. Also watch In The Mouth of Madness, even though it's only inspired by Lovecraft.
I've never read the original stories, but know enough to realize that the Weissmuller films are nothing like the books. regardless, the first two in the series (Tarzan and Tarzan and his mate) are fantastic escapist entertainment.
A lot of great options here. The most accurate thing you will find is the Jeremy Brett television series. They follow the stories very closely, and feature good production values. Many consider Brett the definitive Holmes. I also recommend The Private Life of Sherlock Holmes (a comedy).
Phantom of the Opera
The Silent Lon Chaney version. No question
20,000 Leagues Under the Sea
As others have said, the Disney version is a lot of fun, if a little silly.
Frankenstein and Bride of Frankenstein, although they have little to do with the book. Bride is one of the best sequels of all time, not to mention one of the funniest horror films ever.
Edit - I don't think anyone mentioned Horror of Dracula, the first Dracula film Christopher Lee did. It's a really cool adaptation of the book, with some neat changes that make that movie very fresh.