The Last Man On Earth (1964)
Chosen by me from a list of public domain films on youtube
Matheson's I Am Legend is one of the best horror novels I have ever read but the film adaptations have never quite got it right (I actually like a lot of the Will Smith version but it fucks up the ending big time).
This one comes closest to the book's tone (and to explaining why it is called I Am Legend) but can't stick the landing and is so shoddily made (these vampires make Romero's Zombies in Night of the Living Dead look like world class athletes) it starts to make me wonder what I liked about the book in the first place (only briefly mind - then I recall how awesome it is).
Vincent Price is terrific.