Interesting idea, choosing between different characters of essentially the same hero. Personally I think the main goal for choosing which character to use is to know what kind of film your making. How do you want the team to interact, how does that drive the story and character development, which will get you the best film. A lot of that is in the writing, a lot of it is in the acting, as well as directing.
I never really like Hal Jordan, I just find his character quite annoying. There are only few things I've seen (comics, animation, etc) where I actually like him. The problem is his character is typically the over confident and cocky guy. And there's always a good chance that he can come off as unlikable and hurt the film. Now to be clear I'm not stay that characteristic is bad just a bit tricky. It worked great in the first JL New 52 arc, where he and Batman played off each other perfectly. But fail in the GL movie where you just hate the hotshot pilot who puts an entire company worth of jobs on the line to prove a point and is rewarded with ultimate power and the girl; while hector hammond, a shaper of young mind who deserves respect, gets treated like trash by everyone gets a power that turns into a hideous psychopath. The point is you shouldn't be rooting for the second rate villain. After all is said and done I wouldn't choose Hal except... wait for it... if he is being played by Nathan Fillion, someone who can pull off the cocky hotshot with the charm needed to make him incredibly likable, much like the greatness Robert Downey Jr. achieved. For Hal it's all about the casting.
Now for John Stewart I love his character I thought he was great in the cartoons. I think being a marine and architect is a perfect background for the green lantern corp, I feel like that adds a lot more to the character than test pilot. Plus the way he plays off flash in JLU is fantastic and they made a great duo. The problem is that I feel he is very under utilized in the comics which is a shame since JLU really gave him a lot of good exposure and good ground work to grow. It comes back to what kind of team and character you want, if you want the cocky but charming, or the tough no nonsense soldier. I for one prefer the latter.
For Flash, I honestly don't care that much because I feel that either way their going to combine the best of both so it's going to be about the name and hair color. And I predicted the JL movie will have Barry Allen if only because he is the current Flash. For his personality I really feel they should go with comedic from JLU. Because it comes back to team dynamics and it's good to have a comic relief, now this doesn't mean he'll be over the top, but in JLU Flash had great moments in that show not only as the humor, but as the innocent, trusting, and honest one. Just all around a good guy, (see JLU episode Flash and Substance). It really help define him, make him stand out on his own, but also support the whole team dynamic. If Batman is the Brains, Superman is the Hope, Wonder Woman is the Honor, then Flash is the Kind.
So my choice would be John Stewart and Barry Allen, playing on the GL/Flash role they had in JLU. I think it works really well and to me it's more important to have the best character interaction for the team and I feel GL/Flash is best and Stewart fits that better than Hal. Though again if Nathan Fillion or someone equally perfect for that kind of role I would gladly welcome him.