I've stumbled across a new core that's really been working well for me. It uses the underappreciated Hydreigon. I know it previously had use in the Tapu Lele / Mega Metagross / Hydreigon core as it checked Aegislash and Cresselia and formed the Dragon/Fairy/Steel combo. But my core is different. It's more a Dragon/Dark/Ghost/Poison/Fairy/Water core :p
I realised while playtesting for the Kalos Classic that Mega Gengar and Hydreigon are perfect partners. The combination is crazy. Hydreigon is immune to Ground and Psychic and resists Dark and Ghost. That covers literally every Gengar weakness. Then in return Gengar resists Fairy and Bug and is immune to Fighting, which covers 3 of Hydreigon's weaknesses. The only thing left to cover is the Ice and Dragon weaknesses, which is where the last part of the core comes into it.
Tapu Fini resists Ice and is immune to Dragon, and provides status protection and terrain control. Its weaknesses of Poison, Electric and Grass are all covered by Gengar and Hydreigon.
I have found the Mega Gengar, Hydreigon and Tapu Fini core really fun to use. I currently run Choice Scarf and Wiki Berry on the latter 2 respectively. Still finalizing the Hydreigon moveset because it has so many great options (Fire Blast, Heat Wave, Flash Cannon, U-Turn, Draco Meteor, Dark Pulse, Earth Power). I know Darkinium Z is the standard Hydreigon item, but I hate being outsped by things and Scarf can surprise people (though Hydreigon could benefit from Trick Room or Icy Wind support instead).
I usually run the 3 with Incineroar or another supporter like Celesteela, Hitmontop or Clefable. The core is all special attackers, so you need backup physical options on the team in case you hit a special wall or Chansey
It's worth noting that Heatran and Celesteela make great replacements for Tapu Fini in this core because they resist Ice and Dragon as well. Though Heatran would risk double Ground and Fighting weaknesses in the core which is not as desirable.