I'm torn between Little Women (the version from the 90's with Winona Ryder), and Mr. Blandings Builds His Dream House. I can't decide which one I like better.
I love old movies from the 1940's and 50's and I've recently discovered that quite a few of my favorites from then started out as books. So I've been exploring those a bit - other good ones are Please Don't Eat the Daisies, Yours Mine & Ours (not the new version, but the old one with Lucille Ball which was based on a book called Who Gets the Drumstick), and Father of the Bride (the original with Spencer Tracy, although I do enjoy the remake as well).
Sorry, I didn't really stick to just one, because I get so excited about old movies and the books they were based on that I can't contain myself.