Potassium atom has an outer shell electron that needs less energy to take out than that of Sodium atom. So, I would guess KCL (Potassium Chloride) solution would make the better conductor.
I'm not sure ..but you can start by thinking of which has the least amount of free electrons in the outer shell..that one should be the most conductive.
I would guess (no promises) that it is NaCI, as that is a salt. Salt in a solution typically will cause it to conduct better than other types of solutions.