Well, it depends what the question is. The most legendary of them all, the most iconic is clearly John Wayne. Before I turned cinephile, I wouldn't have been able to tell Cary Grant, Jimmmy Stewart and James Cagney apart. That is, I could identify that there were all old movie stars, but which one ? Well, maybe Grant, and certainly Fonda because of Once Upon a Time in the West... and probably BOgart... but none of them would have been as familiar to me as John Wayne, even having not seen a single one of his films.
Now that I think of it, maybe La classe américaine (which is a parodic film made out of the Warner Bros catalogue with humorous French dubbing) featuring Wayne very heavily plays a role there. But whatever the case, he seems the most iconic of the bunch to me by a (western) country mile.
My favorite now, however ? James Stewart, easily. I do love Grant and Fonda as well, and possibly Hepburn and Davis too though I need to see more from them to really know.