What every type takes damage from, read defensively. Weaknesses double the damage, resistances halve it, immunities cancel it. On a dual type the multipliers stack, so the worst combinations take quadruple damage from a single move.
Dual types that take 4x damage
When both of a Pokémon's types are weak to the same attacking type, the multipliers stack into a quadruple hit. These 53 combinations each have at least one, and the 8 at the top have two or more.
Using weaknesses in a battle
Damage from a super effective hit doubles, which usually matters more than any stat difference between the two Pokémon. A 4x hit on a dual type will knock out most things outright, so the first question in any matchup is what the opponent is weak to.
Immunities beat everything
There are seven immunities in the game and they override any other multiplier. Ground moves do nothing to a Flying type, Electric does nothing to Ground, Poison does nothing to Steel, Psychic does nothing to Dark, Dragon does nothing to Fairy, Normal and Fighting do nothing to Ghost, and Ghost does nothing to Normal. Abilities such as Levitate, Flash Fire and Water Absorb add more on top.
Resistances can cancel weaknesses
A dual type where one half resists what the other half is weak to comes out neutral. That is why some combinations are far sturdier than their individual types suggest, and why counting weaknesses per type is only a starting point. Check the exact Pokémon with the type calculator.