0635 Hydreigon

Dragon Breath and Bite both hit hard with STAB, though the former has significantly better energy gains which makes it the superior choice. 

Brutal Swing is a move limited to Community Day events and Elite TMs, but its lesser energy cost improves Hydreigon's performance and consistency in terms of damage output and shield pressure. Dark Pulse costs noticeably more energy, and it is generally recommended to postpone evolving Zweilous until future Community Day recap events in order to obtain Brutal Swing instead. Dragon Pulse isn't as necessary mainly due to requiring more energy, though it can threaten OHKOs against opposing Dragon-types. Flash Cannon is Hydreigon's only hope at threatening Fairy-type targets in some way, but its expensive cost makes it extremely unreliable.

Disadvantageous typing, slow charged moves, and incredibly low bulk make Hydreigon not really worth running in the Great League. Being hardwalled by the popular Azumarill, as well as any other Fairy-type, is a major risk. 

Hydreigon's typing is certainly less bad in Ultra League than Great, but once again it's held back by its low bulk and vulnerable weaknesses.

Hydreigon shines best in the Master League, although it can be lackluster. Compared to other Dragon-type Pokemon, Hydreigon's stats are somewhat below average. Its Dark typing makes it a great answer to Giratina-Origin, Zarude, and Mewtwo. However, it also curses Hydreigon with a Fighting weakness and an amplified Fairy weakness. Chief abusers of these types include Melmetal, Zacian, and especially Togekiss.

Perhaps the biggest issue with Hydreigon is that it really lacks a powerful charge move to threaten its checks, especially when compared to the nuclear options that other Dragon-type Pokemon have access to. As a result, Hydreigon often cannot seize opportunities to turn bad matchups around. It is stronger in a Premier Cup format where Dragon-type competitors are scarce, although Hydreigon's Dark typing sees less targets there.