The Last Man on Earth -- Scary as hell, this one made me cry when I was a kid. Vincent Price (in a rare non-hammy performance) is the only man on the planet immune to a disease turning everyone else -- his wife, daughter, friends, neighbors -- into vampires. Based on Richard Matheson's novela I Am Legend, this Italian/US production served as obvious inspiration for George A. Romero. You can pick this one up for a dollar. I bought mine at the Christmas Tree Shop for $5.99 and it turned out to be a nice, widescreen cut.
Night of the Living Dead -- Since most people missed The Last Man on Earth, this is considered the seminal zombie classic which launched a genre. The grainy black-and-white film stock gives it an almost documentary feel, which adds to the scariness. You can pick this one up for a dollar, although it's worth it to pay more for a nicer transfer.
Carnival of Souls -- OK, so it's an artsy student film, but unrelentingly creepy. The purposely mixmatched soundtrack and fast/slow motion scenes are like watching someone's nightmare (or your own). This one's at the dollar store and in virtually every PD horror collection.
Creepers (AKA Phenomena) -- This minor US/Italian effort by horror maestro Dario Argento concerns a teen (Jennifer Connelly, back when she was young enough to have an excuse for being a lousy actress) who can control insects with her mind. Abounding scenes of squirming bugs and an out-of-freakin'-nowhere shock ending make this suitably icky Halloween fare. I got my decent transfer on a double-feature EastWest release at Stop & Shop.
The Devil Bat -- For pure cheese, pass up Plan 9 from Outer Space in favor of this living-Lugosi train wreck about a perfume chemist (named "Paul Carruthers," LOL!!) taking revenge on his superiors by breeding a giant bat that attacks people wearing the perfume. The fake bat is hysterical, but not as funny as Lugosi himself, a ham to the very end. I picked mine up at Dollar Tree.
That's my list for now. Anyone else?