pH test kits aren't very accurate...best to use a meter for that. Salifert and Elos are the only ones I would spend money on. Salifert for alkalinity, either for the rest.
Although I would like to use a KZ Potassium kit, but not willing to pay the $40 for an apparently really difficult test to read.
As for which kits to replace first, do you have a FO or reef tank?