This is it. The big one. The film that features dead last on nearly every Pokémon ranking. Disappointing as it may be, I cannot but agree with common opinion here: "Pokémon the Movie: Hoopa and the Clash of Ages" is easily the worst film in the franchise.
The story is that genie Pokémon Hoopa summons a bunch of legendaries and that they fight.
All right, that is not entirely honest; the first half still tries to hold up the pretence of having a story, opening (after the usual redundant prologue) with a Cave of Wonders à la Aladdin. This could have made for an interesting motif (like how "Ponyo" contains elements of "The Little Mermaid,") had the writers done anything with it.
But alas, the story is merely an excuse for the fighting. In my review of "Genesect and the Legend Awakened" I complained that these films aren't adventures anymore, and "Hoopa" is the epitome of this gripe. The main plot literally begins with Ash and Co. hanging out in a resort when Hoopa drops in and just starts messing up things. For some convoluted reason, the genie transports nearly all legendaries from previous films to Dubai (or what is it? Dahara City? Not so much inspired by Dubai as a listless copy of the place) where they start fighting each other. The mere notion of this will probably whet the appetite of any kaiju fan, but I am afraid I like my films to have a certain amount of substance -- motivations, stakes, character arcs and those sort of things.
Apparently, I am part of a minority in this respect. Despite "Hoopa" being consistently ranked by critics as the worst in the franchise, its IMDb score is higher than those of the 5 films preceding it. I can only imagine this is the case because many viewers are content to watch big things punch each other. But the lack of context is detrimental to the entertainment factor. The best of the Pokémon films (among which "Mewtwo Returns," "Latias and Latios," "Lucario and the Mystery of Mew") have exciting climaxes because we have followed their characters on a journey, because we know their reason to get involved in the plot, and because the origins of their conflict is well-established. "Hoopa" doesn't bother with anything. Its spectacle is hollow, its writing lazy and cynical.
The film seems to exist solely to show off the new 'mega evolutions,' allowing for Pokémon to reach a fourth stage in evolution for a limited amount of time after the employment of a certain essence. This is "Pokémon" at its most creatively bankrupt: As if the new monster designs aren't bad enough, the artists now go back to ruin some perfectly fine artwork by adding more bits to it -- sticking extension cords to Rayquaza's face, and the like.
Nowhere do these mega evolutions feel more out of place than amongst the legendaries this film is so proudly parading with. The whole appeal of a legendary Pokémon is its rarity and position at the top of the food chain. Temporarily upping their power level only undercuts the impact they would normally make.
Indeed, diminishing the success of earlier ventures seems to be "Hoopa"'s speciality. I will not go into spoilers, but let it suffice to mention that one of the legendaries the genie summons has died in its own film. Its reappearance is never explained. The writers simply took one of the series' few impactful moments and rendered it meaningless. They even add insult to injury by inserting two of those unbearable fake-out deaths the franchise is teeming with. Incomprehensible that a film focussing solely on fighting cares so little about making an impact.
"Hoopa and the Clash of Ages" surpasses "Vicitini and Zekrom/Reshiram" and "Genesect and the Legend Awakens" in terms of awfulness. It is not only lazy and boring, nearly plotless and cynically conceived, but even attempts to retroactively ruin previous accomplishments. This film should have been a franchise killer; it is a miracle the studio held out for one more before they rebooted the whole thing.