I could go for days, but wanted to get some of the great directors' movies on board. I apologize in advance for any duplicates:

4 directors immediately come to mind:

Alfred Hitchcock:
(dates approximate):

The 39 Steps (1939, Robert Donat, Madeline Carrol, I think)
To Catch A Thief (1955, Cary Grant, Grace Kelly)
Rear Window (1954 maybe? James Stewart Grace Kelly again)

Frank Capra:

Mr. Deeds Goes To Town (1936, Gary Cooper, Jean Arthur)
Mr. Smith Goes to Washington (1939, James Stewart, Jean Arthur again)
It Happened One Night (1934, Clark Gable, Claudette Colbert)
It's A Wonderful Life (already mentioned? 1946)

John Ford:

anything with John Wayne, most especially

The Searchers (1956)
Rio Grande (1950)
The Quiet Man (1953)
Liberty Valance (1962)
3 Godfathers (1948?)

or without John Wayne, like The Wagonmaster (1950)

Howard Hawks:

and John Wayne again: Red River (1948) Rio Bravo (1958)
or without, like The Big Sleep (Bogart and Bacall, 1946 or 8)

Like most of you, I'll think of a lot more sometime soon.
Bill