C- => C+
I remember when we had about half a page of discussion about this thing and it didn't really go anywhere afterwards, so I'm going to share my thoughts on the subject.
Hydreigon has a lot of flaws, we can all agree on that, from it's crippling 4X weakness to one of the most common types right now to a not-too-good 98 speed to being weak to common things like Fighting and Ice. It's not in the best position right now. But putting it in with stuff like Blissey, Torkoal and bloody
Cofagrigus is just a crime, especially when it does so much to the upper tier. It's Life Orb 3 Attacks + Roost set is probably one of it's better one's right next to scarf. Life Orb Draco can do so much on a unresistant hit and Dark Pulse can wear down walls that don't resist it pretty well, it also has pretty good coverage moves in Fire Blast and Earth Power for Toxapex and specially defensive Celesteela.
Time to show some calcs:
252 SpA Life Orb Hydreigon Draco Meteor vs. 0 HP / 0 SpD Landorus-Therian: 324-382 (101.5 - 119.7%) -- guaranteed OHKO
(Defensive Landorus has a chance to be OHKO'ed after rocks.)
252 SpA Life Orb Hydreigon Dark Pulse vs. 0 HP / 0 SpD Metagross-Mega: 307-361 (101.9 - 119.9%) -- guaranteed OHKO
(You survive any attack Metagross can hit you with at full health bar Hammer Arm, Ice Punch has a low chance to KO after rocks.)
252 SpA Life Orb Hydreigon Fire Blast vs. 252 HP / 252+ SpD Celesteela: 218-257 (54.7 - 64.5%) -- guaranteed 2HKO after Leftovers recovery
(This is a fully invested Celesteela and it still gets 2HKO'ed, heavy slam has pretty piss poor chances of 2HKOing you. It gets even better the less specially defensive it is. I know fire blast does at least 75% on physically defensive variants and I think it has a chance to OHKO on sweeping variants)
252 SpA Life Orb Hydreigon Fire Blast vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 221-260 (54.7 - 64.3%) -- guaranteed 2HKO
(Do I need to explain myself here? It kills one of the most bulky pokemon in OU by clicking a move twice)
Looking at all these calcs are impressive and all, but who cares when the opponent can just bring in their Tapu and scare you out? Well about that.....
252 SpA Life Orb Hydreigon Dark Pulse vs. 0 HP / 4 SpD Tapu Lele: 146-173 (51.9 - 61.5%) -- guaranteed 2HKO
(Tapu Lele cannot switch into a Hydreigon using Dark Pulse and is out sped and 2HKO'ed, it has to come in freely or when it switches into a dragon move)
252 SpA Life Orb Hydreigon Fire Blast vs. 0 HP / 0 SpD Tapu Bulu: 317-374 (112.8 - 133%) -- guaranteed OHKO
(Even Max HP AV sets cleanly 2HKO and set lacking Super Power fail to do much to Hydreigon, Stone Edge not even 2HKOing even when fully invested adamant)
But what about Tapu Koko?
252 SpA Life Orb Hydreigon Fire Blast vs. 0 HP / 0 SpD Tapu Koko: 192-227 (68.3 - 80.7%) -- guaranteed 2HKO
(Hydreigon can't risk staying into a already in Tapu Koko, but can predict a switch into koko, do at least 68%, and switch out, even Dark Pulse can do about 40%, which really deters it from constantly switching in)
The only Tapu that Hydreigon struggles to do anything against is Fini, which leads me to my next point.
Hydreigon can pair with many relevant pokemon very well, it forms a solid core with mons such as Mega Metagross and Mega Mawile, covering each others weaknesses defensively and offensively.
It also pairs well with toxapex
Hydreigon also has so much versatility to it, it can run Earth Power for Toxapex, it can run Flash Cannon to 2HKO most fairies and can even run Taunt is you want more of a stallbreaker set.
Now I'm not saying that Hydreigon is some un-tapped potential pokemon or something, it really isn't, hell it isn't that particularly good in SM OU. But the fact that it's on the same level as the mega somehow worse then the normal form, the ghost wall that has no reliable recovery and the turtle who's most viable abuser of it's weather effect is normal Venusaur when it can do all of the stuff above just makes it an un-fair rank in my opinion.
I'm not that particularly experienced in OU metagame, so feel free to correct me if you feel I'm wrong.