Unlike Superman and Batman, these two are honestly friends. Best chronicled in Flash and Green Lantern: The Brave & The Bold, and theyve often teamed up in their own titles, their friendship is one of the most convincingly portrayed in all of comics. Even though the friendship started with Barry Allen and Hal Jordan, the friendship has continued with all iterations of the heroes. Realistically, the best set-up for a cross-over movie would be the JLA movie. They meet there, lets say, and form the friendship. But all future movies with these characters could be benefited from a guest appearance by the other.