Weak toGroundGhostPsychicDark ×1.6

Attacker	Type	Moveset	DPS
Mewtwo	Psychic	ConfusionPsystrikeLegacy	29.3
Blacephalon	FireGhost	AstonishShadow Ball	27.4
Alakazam	Psychic	ConfusionPsychicLegacy	25.0
Hoopa	PsychicGhost	ConfusionShadow Ball	24.9
Darkrai	Dark	SnarlShadow Ball	24.7
Chandelure	GhostFire	HexShadow Ball	24.5
Gengar	GhostPoison	LickShadow BallLegacy	24.5
Lunala	PsychicGhost	ConfusionShadow Ball	24.3
Groudon	Ground	Mud ShotPrecipice BladesLegacy	24.3
Azelf	Psychic	ConfusionFuturesight	24.3
Espeon	Psychic	ConfusionPsychic	24.1
Absol	Dark	SnarlBrutal SwingLegacy	24.0

Windy boosts the Psychic attacks above. Fog and Overcast boost Mega Gengar.

Level 40, 15 attack IV, costed against Mega Gengar's own defense and typing, one per evolution family.

Mega Gengar's moveset, stats and CP All counters Raid board

Questions people ask

What is Mega Gengar weak to in Pokémon GO?

Mega Gengar takes 1.6 times from Ground, Ghost, Psychic and Dark.

What's the best counter to Mega Gengar?

Mewtwo with Confusion and Psystrike, worth 29.3 damage per second against Mega Gengar's own defense. The full ranking is above.