Summoner : Which pet element to choose?

So I've been wondering exactly which element my pets should be and why, does it matter? In very old forums from when summoner was new people said light element. I have played summoner since its release back then and I honestly still don't know which element my pets "should" be.

Generally Light element is the weapon MOST classes wanna go with because the amount of monsters that are weak to Light are in more abundance or more common than others