Ummm I'm not sure. I think they're all well designed and have great backstories. I like that none of them hinge on the plot (aside from Crystal's Suicune, but it's not as intrusive as future box legendaries would become so it gets a pass). That all being said, it's hard to pick one, but I think if I had to make a selection, I'd go with Entei for the 3rd movie. My mom took my sister and I to see it when it was in theaters and we were the only ones there aha. I remember getting the promo and my mom never really indulged in my interests much, so even at 24, I still think of that as a huge thing for her to have taken me to that movie. Entei being the only Pokemon she really knows, it's her favorite, so for all of that, I'll say Entei is my favorite gen II legendary.