Water
Water and Flying types: weaknesses, resistances and Pokémon
Water and Flying Pokémon take quadruple damage from Electric moves, take double damage from Rock moves, resist Fire, Water, Fighting, Bug and Steel moves and are immune to Ground moves.
With 6 resistances and immunities against 2 weaknesses, it is a very solid defensive combination.
Water moves are super effective against Fire, Ground and Rock and not very effective against Water, Grass and Dragon. Flying moves are super effective against Grass, Fighting and Bug and not very effective against Electric, Rock and Steel.
To beat these Pokémon, your best pick is Electric moves. Avoid Fire, Water, Fighting and Bug moves, which deal little or no damage.
There are 8 Pokémon with this combination, such as Gyarados, Mantine, Wingull and Pelipper.