Gyarados

Mega Gyarados is a dominant force in the 1v1 metagame. After it Mega Evolves, Gyarados's ability becomes Mold Breaker, allowing it to ignore abilities such as Sturdy, Multiscale, and Magic Bounce from notable foes such as Crustle, Dragonite, and Mega Sableye. Gyarados may also opt not to Mega Evolve, preserving its Water / Flying typing over Water / Dark, opening up an entire other range of Pokemon it can tank attacks from such as Magearna and Sawk. Additionally, Mega Gyarados has an impressive movepool featuring usable STAB attacks in Waterfall and Crunch, good coverage options such as Outrage and Earthquake, anti-stall measures like Taunt, and also boosting moves, particularly Dragon Dance, which it is an exemplary user of. This is because of its great bulk paired with its high Attack stat, and despite its somewhat lackluster Speed, it's capable of outspeeding most Pokemon with proper investment after a boost. While it is indeed incredibly powerful, it's far from unstoppable, with bulky offensive Pokemon that have favorable type matchups like Tapu Lele, Mega Venusaur, and Mega Altaria countering it.

Gyarados also has access to Supersonic Skystrike to complement its Water typing quite nicely, allowing it to lure in and break through typical Mega Gyarados checks like Mega Venusaur, slow Kommo-o, and Kartana. Flyinium Z Gyarados does have many flaws, however, including the inability to change its typing and the loss of Mold Breaker. This leaves it vulnerable to Pokemon like Dragonite, Golem, Aegislash, and Magnezone because it cannot change its typing. Flyinium Z Gyarados also overrelies on Bounce and Supersonic Skystrike, which means that once its Z-Move is spent, it has to use its unreliable two-turn Bounce, which can easily be taken advantage of, particularly by Protect and Substitute users like Serperior.