Oddly enough, another "counter" to Hydreigon is Specially defensive Celebi, if rain is up. Celebi avoids the 2HKO from Draco Meteor, recover on Hydreigon's face, and Fire Blast will do laughable damage thanks to rain. However, like Jellicent, Celebi will fall to Dark Pulse. Also, unlike Jirachi, another Pokémon that can be considered a "counter" if rain is up, Celebi cannot do much to Hydreigon, unless it carry Hidden Power Ice or Thunder Wave, but the former can only 3HKO and the latter only manages to cripple Hydreigon, not defeat him. Also, now that you mentioned it, Kyurem-B lacks physical coverage moves to hurt Steel-types (Fusion Bolt covers most Steel-types neutrally, but isn't sufficient to do a lot of damage to most them. This doesn't happen with Hydreigon, so Hydreigon could be considered A-Rank? (or Kyurem-B, B-Rank)?