Of all the Pokémon listed for this event, I feel Jirachi is probably the one people would get the most use out of. It's been released fewer times than Victini, Manaphy, and Darkrai. Two of whom have distribution ROMs running around still, and one you can put a legal Wondercard onto your game card via editing.

To vote: Scroll down to the keys. Click and drag the key of the Pokémon you want to vote for to the key hole below, then select the button on the left once the popup comes up.

If this is anything like the last events, the Pokémon that wins in Japan will be what everyone else gets.

Vote here: http://www2.pokemon-movie.jp/tickets_pokemonVote/