I sent this list to a friend for their radio show, and I consider these films to be required watching for any Halloween movie marathon.
Night of the Living Dead, both the original 1968 classic, and Tom Savini's 1990 gore-fest. The latter starred Tony Todd as Ben. He would later go on the play the titular monster from the Candyman movies, as well as various roles in different Star Trek and Transformers properties!
Return of the Living Dead, which is the ultimate 1980s scary movie. A perfect blend of comedy, horror and more 1980s pop culture than you can shake a Colecovision controller at. Its also the film that started the fast moving, brain eating zombies craze.
The 1931 Spanish Language version of Dracula. Filmed in tandem with the Bela Lugosi version, and using the same set pieces and costumes; the Spanish language version's crew were able to watch the dailies of what the English language version's crew had shot earlier. This allowed them to make improvements in pacing and exposition.
Dracula Dead and Loving it. One of Mel Brook's best parodies. It recreates some scenes from Dracula shot-for-shot, while lampooning others from the Francis Ford Coppola remake. It has a star studded cast, including Leslie Nielsen as Dracula, Peter MacNicol doing fantastic rendition of Dwight Frye's Renfield, and Mel Brooks himself as Professor Van Helsing.
Plan 9 From Outer Space. Dubbed by Rifftrax as the "Citizen Kane of bad movies", it certainly lives up to that title. Ed Wood sought to make a suspenseful sci-fi shocker, about grave robbers from outer space, and instead created one of the greatest comedic travesties ever captured on film. A movie so bad, that Bela Lugosi decided that upping and dying would be more preferable to starring in it! ...too soon?