Prolly too simplistic, but Victor Spinetti comes to mind first.
As a master of Beatles trivia (humbly proclaimed), I would have said the same thing. Spinetti was a
brilliant addition to all three movies he appeared in. But, it was recently brought to my attention that someone else holds the record with four (4) appearances. I had to check it out and - yep, "one" "two" "three" "four" (Can I have a little more) - it's true. However, one appearance is so brief, you have to pause your DVD player to confirm it. Any other guesses?
I love learning new stuff about the Lads.