Argon-40 because it is the isotope with the closest mass to the atomic mass of the element. Making it more stable than the others allowing it to be the most abundant in nature.
The relative atomic mass of Argon is 39.948. This is very close to the mass of argon-40. Usually, the relative atomic mass of an element is the sum of the percentage abundance of each isotope in nature multiplied by its mass. The result of this sum is normally biased towards the isotope with the highest abundance. In the case of argon, argon-40 must have the highest abundance, hence the value of the relative atomic mass.
