Difficult one, perhaps "Rebecca". Or is it "Dial M for Murder"?
The best "Hitchcock" movie that was NOT directed by Alfred Hitchcock, but clearly in his style, is "Charade" by Stanley Donen, featuring Cary Grant and Audrey Hepburn.
Brian de Palma was clearly influenced by Hitchcock, even almost copied a few scenes in some of his movies. ("Dressed To Kill", "Body Double").
A very good and underrated movie is "Peeping Tom", directed in 1960 by Michael Powell. The idea of the movie (a young man killing women with the filed leg of his camera's tripod and filming them in the act of being killed) was too shocking for the audience of that time, and so this excellent thriller was almost forgotten when it was at last revived in the 80s.
Also an excellent thriller: "Wait Until Dark", directed by Terence Young, featuring the excellent Audrey Hepburn.
Oh, and let's not forget the excellent "Sleuth", featuring Lawrence Olivier and Michael Caine. What a duel between two men! And let's not forget the excellent performance of Alex Cawthorne as Inspector Doppler in this movie. He acts as if he isn't there!
Edited by BaldFriede