While I'm not a huge fan of John Stewart, I'd take a movie focusing on him and this Corey Reynolds dude playing him in a heartbeat before I'd sit through Jack Black farting green smoke clouds all over the screen. I'd prefer a movie focusing on Hal, but just from a social aspect, I can understand if DC/WB wants to make a superhero movie focusing on an African American character, then they might as well actually USE one of their African American characters instead of doing something like making Will Smith the new Superman (yes, there was actually a possibility of this happening once).
Personally, for me, I'd like to see a movie based on the GL corps, with Hal as the main character in the first film, and have Guy and John also be in it. I'd introduce Kyle in the sequel, though I don't know if I'd go the whole "Parallax" route, simply because what made the whole "Hal turning evil" thing so shocking was that he was a character with decades and decades of history that we'd all seen unfold. But those were comics. To movie audiences, they'd see him get his ring and save the galaxy in the first film, only to turn bad in the second? It just doesn't have the same impact. The reaction would be more like, "uh... what?"
As for an Alan Scott film... that would be a cool route to take too, though not the one I'd prefer. But I would like to see one of these superhero films set back in the time of the hero's original creation. Setting a film in the 40's or 50's adds a really cool element to it... for example, the film Dick Tracy wasn't really all that great, IMO, but the set designs were awesome. Imagine how cool a movie about the Sandman (the gas mask one, not the Neil Gaiman one) could be, if set in the 40's.