What punctuation should be used in the question Which impact was harder first or second?

The correct punctuation for the question "Which impact was harder, first or second?" is a comma after "harder".

Here's why:

* "First or second" acts as a parenthetical phrase that provides additional information about the impact being compared.

* Parenthetical phrases are typically set off by commas.

Therefore, the complete sentence is: "Which impact was harder, first or second?"