Frankenstein (1931) and Bride of Frankenstein (1935). Boris Karloff's stellar performance as Frankenstein's creation/monster still can't be beat, and the moody black and white photography and James Whale's eerie, dream-like direction contribute, especially in Bride, to suggest a bizarre world where anything horrific can occur. Universal Films' various sequels are okay, but except for 1942's guilty pleasure, Frakenstein Meets The Wolf Man (with Bela Lugosi as the monster and Lon Chaney, Jr.), lack a certain sort of oomph.
Boris Karloff without the Frankenstein connection was pretty good on his own too. Check out the Icons of Horror collection that highlights four of his best films (including the great, ironic classic from 1935, The Black Room).
The Hammer Horror Films: Beginning with 1956's The Curse of Frankenstein (see the image on top of this entry) and 1958's Horror of Dracula, this suburban based (out of Bray) Britsh studio cranked out some impressive and memorable thrillers, notable for their then-innovative use of color, gore and, urr, sex. They also managed to make stars out of leading men Peter Cushing and Christopher Lee as well. Check out the aformentioned Frankenstein and Dracula films as well as the DVD collections The Hammer Horror Series (w/ 1960's The Brides of Dracula and 1962's Phantom of the Opera, both sublime classics) and Hammer Films (which I reviewed here a while back).
B-film director Roger Corman, working from a script by fantasy/horror specialist Richard Matheson (I Am Legend) and casting Vincent Price as the star, produced two mini-masterpieces based on the works of Edgar Allen Poe: 1960's The Fall of The House of Usher and 1961's The Pit and the Pendulum. Both are available on one DVD here.
Italian director Dario Argento delivers the shocks with his films The Bird With The Crystal Plumage (1969), Suspiria (1977) and Tenebrae (1982). Warning: These otherwise exciting, thrilling and scary thrillers (which have influenced the likes of John Carpenter, Sean Cunningham, John Landis, and the entire run of the various CSI TV shows) are quite violent. Not for kids.
Argento himself was influenced (and briefly mentored by) the legendary Mario Bava, whose 1960 gothic masterpiece Black Sunday, is still a tough act to follow. Check out what the library carries here and here. Bava's later work, such as 1963's Black Sabbath (with Boris Karloff!) and 1973's Lisa and the Devil, are also powerful haunting pieces as well.
I'm sure I left out somebody's favorite, but in the meantime, I hope I've given a good idea of the good stuff that's currently available to watch. Harry Halloween!