What do you guys think would become the most effective Hydreigon sets once we get access to Draco Meteor?
Imo, the most common sets would be:
Counters: Chansey, Blissey, Calm Milotic, Calm Mantine (if no shock wave), Calm Togekiss, Careful Mandibuzz, Probopass, Umbreon, Porygon2, Lickitung, Lickilicky, etc.
Counters: Calm Milotic, Aerodactyl, Calm Volcarona, Careful Scizor (if no Flamethrower), Lickitung, Lickilicky, Illumise (if no Flamethrower), Slowking (if no Dark Pulse), etc.
Counters: Calm Milotic, Chansey (if no work up), Calm Mantine, Careful Mandibuzz, Calm Togekiss, etc.
There are plenty of other sets for Hydreigon, but imo if someone chooses to not run Calm Milotic, there is a decent chance the pokemon that switches on Hydreigon will die for no other purpose than scouting Hydreigon's set. Bold Mantine, Bold Milotic, Careful Hippowdon and several other mons that used to check Hydreigon are no longer able to do so by giving Hydreigon access to Draco Meteor.
Volcarona is in no way comparable to Hydreigon. There are way too many checks and counters to Volcarona that beat the vast majority of Volcarona's sets. Hydreigon counters do not counter the vast majority of Hydreigon's sets once it gets Draco Meteor. Volcarona always ends up with serious coverage issues depending on the set it runs unlike Hydreigon, so it's very simple to build an effective defensive core to stop it.