Slowly marching across a busy city amidst the raging Sandstorm that it had summoned, while letting out its
loud cry and causing immense collateral damage with every casual swing of its mighty tail, as well as its every step which trembles the very Earth, the Armor Pokémon is soon confronted by the sound of
an equally intimidating cry. Raising its head to face the source of the roar, the Rock/Dark-type Pokémon then witnesses the Brutal Pokémon descending towards it from the heavens, with each of its three heads roaring loudly and biting the air insanely, expressing nothing short of utter fury and an overwhelmingly strong desire to utterly obliterate its newfound opponent! Undaunted by the ferocious three-headed monster that had just appeared before it, Tyranitar then lets out its own cry, signaling its acceptance of the challenge it had just been issued by the Dark/Dragon-type Pokémon! Upon landing on the ground before its fellow Dark-type Pokémon, Hydreigon then roars loudly at Tyranitar with all three of its heads, provoking the Armor Pokémon to begin the battle by firing its devastating Hyper Beam! After briefly charging a sphere of brilliant golden energy within its mouth, Tyranitar then unleashes its destructive Normal-type attack at the dark hydra, which swiftly responds by pointing all three of its heads at the Rock/Dark-type Pokémon and simultaneously blasting three Dragon Pulses at its direction! Hydreigon's three Dragon-type attacks then collide with Tyranitar's Hyper Beam, and due to the Dark/Dragon-type Pokémon's superior Special Attack, the former attack slowly but steadily overwhelms the latter, hitting the Armor Pokémon straight in the face, and creating a huge explosion which completely topples the pseudo-legendary Pokémon of Johto, which crushes several buildings behind it as it comes down! Watching its opponent unleashing a loud, painful roar as it falls, the three-headed monster then wastes no time as it swiftly flies towards its fallen foe, which now lays amidst a blinding cloud of dust.
While struggling to get back on its feet, with its vision completely covered by the cloud of dust it had raised with its fall, Tyranitar then suddenly sees three draconic heads extending out of the dust in front of it, before biting onto its right arm, left arm, and throat. Hydreigon then further excruciates the dinosaur-like Pokémon by using Fire Fang on Tyranitar's right arm, Ice Fang on its neck using its own main head, and Thunder Fang on its left arm, all while slowly ascending into the air, lifting the Rock/Dark-type Pokémon up from the ground and carrying it up into the sky, as it watches its helpless opponent roaring and struggling in desperation. After the dark hydra reaches a certain level of height, the Armor Pokémon temporarily regains control of its body despite the damage it had suffered, and is still suffering. Taking this opportunity, Tyranitar then attempts to free itself from the grasp of Unova's pseudo-legendary Pokémon, by preparing to strike it with its devastating, super effective Superpower! But in this very situation, the Rock/Dark-type Pokémon pays dearly for its low Speed, as the Brutal Pokémon quickly notices the comeback attempt of its captive prey, and immediately fires a Hyper Beam from each of its three heads while still biting Tyranitar, blasting it with immense power at point-blank range while instantly interrupting its Superpower technique! Letting out a loud cry of excruciating pain, the great pseudo-legendary Pokémon of Johto temporarily disappears into the blindingly bright golden energy of Hydreigon's three devastating Hyper Beams, as pieces of its shattered rock armor fly out of its body. After the light of the three beams fade away, Tyranitar is then seen falling from a great height, before landing hard on the city below, smashing an immense crater on the floor as well as raising an enormous cloud of dust.
From high up in the heavens, the three-headed monster then raises all three of its heads, which each let out a loud, proud, prolonged and bloodcurdling skyward cry in anticipation of victory, before focusing its attention back on the dust cloud-covered crater in the middle of the city below. As it focuses on the aforementioned, the dark hydra then forms a powerful sphere of compressed fighting spirit energy in front of its three heads, before firing its Focus Blast straight into the cloud of dust below, creating a huge, bright explosion that is also accompanied by the Armor Pokémon's death cry.
In short: I prefer Hydreigon. Though Tyranitar is very cool too.