Johnny Quest (my top choice)
Johnny Quest virtually cries out to be made into a live action film. This should definitely be done and soon.
I shy away from Bird Man, Space Ghost, and Mightor, because they all strike me as strongly in the Superman mold, and I'm never satisfied with any Superman film. (No, not even the Christopher Reeve films.)
I think maybe the Galactic Trio could succeed as a live action film. They aren't quite in the Superman mold. They're more like the Three Musketeers. And that's how I would envision the film: Three Musketeers in Outer Space.
What are your thoughts?
Agreed about Jonny Quest. It certainly has the highest popularity.
I think Space Ghost Coast to Coast might have actually hurt Space Ghost's image for a live-action film.