Since movies like having some world-threatening problem, Yellow, GSC and RS are probably the most likely candidates, since both of those have some kick-ass destruction and a really neat final battle. The thing about RGB , FRLG and Emerald is that none of them have some kind of life-threatening situation, and that FRLG ends on a cliffhanger that Emerald picks up on...Emerald makes no sense without FRLG. Yellow and GSC also can't stand alone, since they have continuance from previous sagas, so RS would be the best movie candidate. It even has some definitive romance, unlike the other sagas.