Gil Jawetz

Has Force of Evil been mentioned yet? That's a great one.

gcbma

I'd wait on Forbidden Planet, though. I hear rumored that Warner is intending to re-release it as a 2-disc special edition sometime in the near future.

Regarding the original question, as pointed out, the field is too wide open for pre-1960s movies in general. One of the absolute musts in terms of general appeal is Warner's 2-disc special edition of "Casablanca". From there sample into other genres:
Film noir: "Out of the Past", "Postman Always Rings Twice", or "Big Sleep", e.g.
Suspense: "Strangers on a Train", "Notorious" or "Third Man", e.g.
Comedy: "A Night at the Opera" or "Some Like It Hot", e.g.
Drama: "Treasure of the Sierra Madre", e.g.
Classic monster or 50s-era sci-fi: "Bride of Frankenstein" or "The Thing", e.g.
Musicals: "Singin in the Rain", e.g.
etc., etc.

gcbma

Good suggestion. This is a geat movie for general appeal, too. Universal's single-disk release of this movie was terrific; it includes a good commentary and a few other special features, to boot.