Dark Pulse. Foul Play is more based on the opponent's Attack, and is useless against Mega Mewtwo Y. Since Yveltal is a Mewtwo check in ubers, Dark Pulse not only obliterates a Mewtwo Y but also does a good amount of damage to Mewtwo X, as seen here:
252 SpA Life Orb Dark Aura Yveltal Dark Pulse vs. 0 HP / 4 SpD Mega Mewtwo X: 227-269 (64.3 - 76.2%)
Here is a Yveltal moveset:
Yveltal @ Life Orb
Nature: Hasty (^Spd vDef)
EVs: 252 SpAtk, 252 Spd, 4 Atk
Dark Pulse
Focus Blast
Sucker Punch
Oblivion Wing
