The region with the best legendary Pokémon is definitely the Unova region. Reshiram is my favorite legendary of all time because of its sheer power, beauty, and lore about how it strongly represents the truth. Zekrom is great, mainly because of it being the foil to Reshiram, menacing appearance, its sheer physical power, and how it stays true to its ideals. Kyreum is great based on how it represents being the "empty shell" of Reshiram and Zekrom because it doesn't represent any truth or ideals, but instead just wants to be whole again. The fact that you can fuse the already powerful Reshiram or Zekrom with it and make it even stronger than it already is, which is very strong, makes it even better. In addition, the "Forces of Nature" are great for being mainstays in the competitive scene, and the Swords of Justice have great lore as well, where they basically race throughout the region and act as superheroes for other Pokémon, which is lit. As for mythical Pokémon, generation IV introduced the best ones there. Darkrai was a monster before Game Freak nerfed him for having the overpowered move, Dark Void, a move that put all Pokémon on the opposing side of the field to sleep with 80% (or 85%, I don't really remember too well) accuracy. In addition, opponents would also lose health because of his ability Bad Dreams, which made Darkrai the Freddy Kreuger of Pokémon. Shaymin's Sky Forme is also a monster competitively as well. Of course, how can you forget the almighty deity Arceus, the strongest Pokémon in the game, even more so than Crowned Zacian because of its stats, ability, and multiple ways to play it.