Mega Gengar is a Ghost/Poison type Pokémon, which makes it weak against Ground, Ghost, Psychic, and Dark moves. Therefore, the best counters for Mega Gengar are Pokémon that have moves of these types. Some of the strongest Pokémon to use against Mega Gengar include Calyrex (Shadow Rider), Mewtwo, Deoxys (Attack), Hoopa (Unbound), Primal Groudon, Shadow Mewtwo, Mega Latios, Mega Tyranitar, Mewtwo, Alakazam, Gardevoir, Gallade, and Espeon. It is important to note that Mega Gengar is resistant to Grass, Fairy, Bug, Poison, Normal, and Fighting-type attacks, so it is best to avoid using Pokémon with these types of moves.