I think what PK is trying to say is that Drizzle makes Fire Blast and Flamethrower unviable, so if permanent rain weren't a thing then Hydreigon's place in A-tier wouldn't be a question. SpDef Jirachi and SpDef Celebi are both quickly ended by Flamethrower/Fire Blast outside of rain, and Jellicent simply can't stand up to repeated Draco Meteors (I'm still a firm opponent of running Dark STAB when you've got Dragon STAB, Hydreigon's coverage, and Justified Pokémon running around). Fun fact: Modest Expert Belt Fire Blast in the rain deals a minimum of 73% to standard Ferrothorn. The threat of Superpower and Focus Blast are also huge deterrents for Ferrothorn to even switch in.
Again, unfortunately, he's so goddamn easy to check that I'm undecided on his place in A.
FaceFaceFace: "Genesect is like the Terminator. Scary when he's coming after you, absolutely lovely with ridiculous punch-lines when he's on your side."