By far Spell of the Unown. No contest. Of all pokemon movies it is the only one I can say has an actually decent well-written story. A little girl loses her father, and unable to cope with her loss and upon finding the unown she creates a fantasy world that keeps her blissfully unaware of the harsh reality. Throughout the movie she learns she can't just keep hiding from the truth and must face reality. And instead of the usual "ASH MUST SAVE THE WORLD", no, he just wants his mother back. It's much more personal so you're much more connected to the movie, unlike the other movies where some random legendary-pokemon-filled shit happens and Ash suddenly feels obliged to be the hero for no reason. What I've also noticed is a lot of movies don't really have any real pokemon battles outside of the intro maybe. This movie has multiple, and they're very well done and very memorable. Seriously, who can forget the underwater battle, or the fucking Charizard vs Entei battle? Best movie.
