If there are some event Pokemon that have special requirements to get them they don't need to be taken into account. I'd love to know what are the newest versions to get (at least almost) all different Pokemon. For example if versions A and B together have Pokemon that are in version E, version E doesn't need to be mentioned. I'd like to get a list of Pokemon games in importance order, and importance order means the amount of unavailable Pokemon it covers compared to the 'main game'. Start with US/UM and then add latest versions as possible.
EDIT: The purpose would be then trade/migrate caught Pokemon to the newest version.