Greer Garson in CHIPS is one of those right on the cusp, like Eva Marie Saint in ON THE WATERFRONT or Patricia Neal in HUD. Definitely the largest female role, but substantially less screen time than the male star. I can see solid arguments either way. Garson is very good in the role, and her influence remains after she's no longer on screen.
Because GWTW is such a long movie, Olivia De Havilland has lots more screen time, but Melanie is second fiddle to Scarlett. Scarlett, Rhett, and Ashley are the starring roles (even though Melanie probably has more screen time than Rhett). For me, Garson would lose to Leigh in the lead category or lose to De Havilland in the supporting category.
It hadn't occurred to me that David Niven's role in THE DAWN PATROL might be considered a lead, or Billie Burke in MERRILY WE LIVE, but those arguments could be made, too. So far in our 1938 polls, Mr. Niven has won a supporting actor award and a couple of lead actor awards.
By the way, I read recently that JUAREZ got that title because Paul Muni insisted on it and had the star power to make that happen. That inspired Bette Davis to get WB to change the title of the movie THE KNIGHT AND THE LADY, which eventually became THE PRIVATE LIVES OF ELIZABETH AND ESSEX. If Brian Aherne had had the clout, JUAREZ might have been MAXIMILIAN or EMPEROR OF MEXICO or some such title.