Destiny Deoxys is my least favorite and gets like a 4.5/10.
Funny how preferences differ. Destiny Deoxys is my favourite among the movies. It's one of the few where the fate of the world isn't at stake, it has some neat characters, and Ash isn't the "chosen one" here. He's just another bystander who happens to be in the way when a rampaging Rayquaza seeks out Deoxys. It doesn't feel as forced as the other movies, where any- and everything revolves around Ash and friends.
Many view Destiny Deoxys as "random legendaries randomly fighting", but one has to understand Rayquaza's motive here. One has to start at the beginning. Rayquaza is the king of the sky. Nobody, ever, touches Rayquaza or interrupts him when flying. Pokémon like Pidgeot and Skarmory hide in their nests when Rayquaza flies overhead. Heatran and Garchomp dig a little lower into the ground when hearing his roars. Zapdos, Moltres and Articuno wisely stay far away from him. Rayquaza is boss, and he knows it.
Then one day, Deoxys bumps into him, taking no note of this ruler of the atmosphere. Rayquaza decides that this insult will cost dearly, and charges. And to his great surprise, this offender puts up a fair fight. In the end, Rayquaza's fiercest attack obliterates Deoxys, and Rayquaza flies away somewhat satisfied.
But Deoxys awakens, and Rayquaza, eager for a fight, seeks it out. After all, Deoxys hurt him last time. It's time for a rematch, and this time, Deoxys will learn to stay out of the atmosphere for good.
However, a grain of doubt is planted in Rayquaza's mind during the match over LaRousse city. Deoxys doesn't seem to be the challenger Rayquaza perceived it as, and it appears to care more for his friend than for battling. In the end, Rayquaza understands that Deoxys is a worthy opponent, but not a direct threat. They leave LaRousse together as friends.