The first name I thought of was Clark Gable. But Charlie Chaplin and Douglas Fairbanks could also lay claim. Also perhaps Valentino, for a short period.
There have been some really good guesses, some really crappy guesses, and at least one that made me laugh (Richard Kiel :p.)
The person that the author was referring to, and the very first and only name that popped into my head was . . . (do not read if you still want to guess): John Wayne
Arguable but silly.
No way! Just no way.
Sure he was big and everybody still knows who he is; but plenty of those suggested were bigger than he.
Cary Grant, for example 
This thread has been done before, and the consensus then was the same one as in your spoiler.
Also my vote.
Another vote for John Wayne. His was the first name to pop into my mind, and in my opinion, he can’t be touched. Long live The Duke.
I think, overall, it has to be John Wayne. Long career, some very great films, and the ability to draw in audiences for decades.
Cary Grant would be a second choice.
If we’re going for the jokes, then it’s Darlene Cates (who wasn’t bad as an actress).
So did the author provide a definition of “biggest”? (i.e., how did the author determine that that person was the biggest movie star of all time?)
My quarrel with this choice is simply that he made a lot of so-so pictures which only his fans would feel were worth watching just to see him. Cary Grant, in my opinion, provided entertainment even in bad pictures for people who wouldn’t go specifically because he was in the picture.
Mary Pickford.
No, it was just a couple of unimportant remarks. He didn’t seem to want to nerd-out about it.
Harrison Ford.
Regards,
Shodan
My instinct was Marlon Brando.
And the 54th post mentions Gregory Peck
Andre the Giant
Hmmmmm . . . How about Valentino?
(Though I still want it to be Gary Cooper.)
Wasn’t John Wayne the #1 box office draw for four decades?
Marlon Brando
{{{hugs you}}}
And I’ve never even seen those movies!
Drat! that was mine
Brian