I thought this time would never come...a good time to nominate Hydreigon for B Rank again.
The intruduction of Fairies and the special nerf did hurt Hydreigon's viability a bit, but the Dark Typ Buff allows it to spam that Dark Pulse without much worry. What makes Hydreigon so threatening is, that it has no 'true' counter thanks to its insane movepool. In other words, nothing really can safely switch into that thing. Looking at Hydreigon's stats it has decent speed and bulk but great offense which allows it to go mixed as well.
Special Moves: Draco Meteor, Dragon Pulse, Dark Pulse, Earth Power, Fire Blast, Focus Blast, Flash Cannon, Surf
Physical Moves: Outrage, Crunch, Superpower, Earthquake, Iron Tail, Head Smash, U-Turn
Other Notable Moves: Thunderwave, Roost, Substitute, Work Up
Not to mention, Hydreigon can pull of a great number of different sets: Specs, SpecialScarf, Expert Belt, MixScarf, Life-Orb, Bulky Attacker.
I am sure there are more, but basicly it is really easy to fit into a team and it does its job realiable enough.