When I do a movie marathon, I always try to keep to a certain type and genre. Sometimes I will mix things up a little for sport.
My suggestions:
Nosferatu (1922)
Creature from the Black Lagoon (1954)
Dracula (1931)
The Wolfman (1941)
Frankenstein (1931)
Bride of Frankenstein (1935)
Rebecca (1940)
Shadow of a Doubt (1943)
Psycho (1960)
Rear Window (1954)
The Birds (1963)
The Haunting (1963)
Rosemary's Baby (1968)
The Exorcist (1973)
The Omen (1976)
The Amityville Horror (1979)
The Texas Chain Saw Massacre (1974)
Halloween (1978)
Friday 13th (1980)
A Nightmare on Elm Street (1984)
Hellraiser (1987)
Invasion of the Body Snatchers (1956)
The Fly (1958)
The Blob (1958)
Night of the Living Dead (1968)
Alien (1979)
Bram Stoker's Dracula (1992)
Mary Shelley's Frankenstein (1994)
Sleepy Hollow (1999)
Interview with the Vampire (1994)
Sweeney Todd (2007)
Dawn of the Dead (1978)
The Thing (1982)
The Blob (1988)
The Fly (1986)
The Mist (2007)
The Shining (1980)
The Sixth Sense (1999)
Heavenly Creatures (1994)
Se7en (1995)
Silence of the Lambs (1992)
Evil Dead (1981)
Evil Dead II (1987)
Army of Darkness (1992)
Shaun of the Dead (2004)
Death Becomes Her (1992)
Nightmare Before Christmas (1993)
Ghostbusters (1984)
Young Frankenstein (1974)
Beetle Juice (1988)
The Frighteners (1996)
And I think a marathon should mean eight to ten hours worth of movies. Six hours is just to short to show much of anything!