One of my favorite suspense films is The Spiral Staircase (1946, directed by Robert Siodmak). The wonderful cast includes one of my absolute favorites, Ethel Barrymore, Dorothy McGuire, George Brent, Kent Smith, Rhonda Fleming, Gordon Oliver, Elsa Lanchester (magnificent as always), and Sara Allgood.
The suspense, directing, and acting in this film are sheer perfection. This is the type of movie that makes you fall in love with classic films all over again and I simply could not recommend it enough.
You can watch The Spiral Staircase above or on YouTube. You can also find The Spiral Staircase on dvd or Blu-ray (Amazon link)
George Brent, Kent Smith, Dorothy McGuire, Ethel Barrymore