I’ve been adding the odd name to the Frequent collaborators section on the Alfred Hitchcock article on Wikipedia for a while now, so I decided recently to do the job properly and analyze the “Complete Cast and Crew” pages on IMDB for the 53 major Hitchcock films.
The final data showing everyone who appears 3 or more times can be seen in this Excel spreadsheet.
Quite often, Leo G Carroll is named as being the actor to appear most frequently in Hitchcock’s films, but that honour actually goes to Bess Flowers who appeared in uncredited roles in 7 Hitchcock films. Flowers was an incredibly prolific actress with over 700 roles in just 43 years (according to her entry on the IMDB), which earned her the nickname of “The Queen of the Hollywood Extras”.
Most of the other actors who frequently worked with Hitchcock did so in his British films (i.e. up to “Jamaica Inn”), with Clare Greet, John Longden, Hannah Jones, and Frederick Piper all appearing in at least 5 of the major films.