Step 1: List the natural greenhouse gases and their abundance.
Water vapour, carbon dioxide, methane and nitrous oxide all absorb infrared radiation and warm the atmosphere naturally.
Step 2: Compare their relative concentrations.
Water vapour is by far the most abundant of these gases in the atmosphere, since evaporation from oceans and land constantly replenishes it.
Step 3: Weigh abundance against individual strength.
Even though carbon dioxide and methane matter a great deal, water vapour's sheer quantity makes its overall contribution to the natural greenhouse effect the largest.
\[ \boxed{\text{Water vapour}} \]