There is no definitive answer to your question, Sandy. There are now so many franchise films in so many genres that keep certain actors well and truly in the public's attention. They could probably all lay claim to being the most famous?
Perhaps looking at the wealthiest indicates the most revered in movies or better still the highest fee for recent movies. In the past there would have been highest paid actors such as Clint Eastwood, Nicholas Cage, Harrison Ford etc.
I think she's more iconic than James Dean, tigger. I'd hazard a guess that a lot of young people today haven't a clue who James Dean was - but I'd bet my boots they know who Marilyn was.
Do you know if Marilyn was as famous in life as she was in death? She's an icon, but I'm wasn't around back then, so I have no idea how embellished by her young passing her legend is (just like James Dean, Bruce Lee etc).
I suspect that during their lifetime, Charles Chaplin was more famous, partly due to Hollywood being less swollen with celebrity than it subsequently became.