ElectricGhost
Electric and Ghost types: weaknesses, resistances and Pokémon
Electric and Ghost Pokémon take double damage from Ground, Ghost and Dark moves, resist Electric, Poison, Flying, Bug and Steel moves and are immune to Normal and Fighting moves.
With 7 resistances and immunities against 3 weaknesses, it is a very solid defensive combination.
Electric moves are super effective against Water and Flying and not very effective against Grass, Electric and Dragon. Ghost moves are super effective against Psychic and Ghost and not very effective against Dark.
To beat these Pokémon, your best pick is Ground, Ghost and Dark moves. Avoid Electric, Poison, Flying and Bug moves, which deal little or no damage.
There is 1 Pokémon with this combination, such as Rotom.