who is the biggest movie star of all time ?

All very big names you mention. As the question is biggest movie star of all time (superlative!) I felt even mentioning more than one was watering it down to some extent.

Also it says biggest movie star and not best actor. Kate Hepburn and Meryl Streep would top my list of best actresses ever hands down, but I'm not sure they are the biggest stars, which seems to call for a certain cult. Both Monroe and Hepburn had and still have a substantial cult following. Their posters are still being sold to grace the walls of fans world wide. Surely that is real stardom? Those two ladies (and also Cary Grant) sold tickets at the box office. The absolutely wonderful Kate Hepburn, on the other hand, was at one time labelled 'box office poison'...

With the men I may be a bit out, but somehow Cary Grant seems to be generally considered one of the great stars of Hollywood, whereas both Bogart and Stewart were fine actors (better ones, in fact, than Grant), but somehow don't seem to have quite the same "star" qualities. Don't know if it's down to the roles they played, looks, lifestyle, or something else.