But they shouldn't be going based off popularity, since no Green Lantern is really "popular". Hal Jordan is largley unheard of, and John Stewart is slightly less unheard of.
I mean, it would be one thing if Hal Jordan was the most obscure character in comics and John Stewart was a household name, but the two hold a very minor difference in popularity. And with Jordan being the main GL used in all of DC's upcoming animated films (New Frontier and Green Lantern/Green Arrow in particular), the gap will close even more.
I've argued this a thousand times, and I dont want to really get into it again, so all i'll say is that Hal should be the main character because it's the logical starting point. His origin suits the introduction of the GL concept better, and since he comes before the others, this allows sequels to cover nearly all of them.
Just think about it for a second. If you start with Hal, you can always do a movie with John or Kyle next, but if you start with John or Kyle, the opportunity for a Hal Jordan movie is pretty much gone (unless you wanna do a prequel which just seems like a dumb idea).
As for casting, Nathan Fillion would be the ideal Green Lantern.
Can anyone honestly disagree?