Hasty - If you want a fast Hydreigon without dropping out in both Special Attack or Attack.
Lonely - If you want to add a bit more Power to Crunch and Head Smash, without weakening FB and Draco Meteor.
Mild - If you want to boost up Fire Blast and Draco Meteor, without cutting down on it 2 attacking stats
The EVs should be adjusted accordingly to what Pokémon you'd like to outspeed.
252 SpA/ 252 Atk/4 Spd - This is a very basic EV spread.
196 Atk / 60 SAtk / 252 Spd - Run with Hasty if you want Max Speed.