BCY ML-60 is the best in my opinion. You apply it all over the string/serving and you don't rub it in with your fingers. You use a piece of leather. The friction from your fingers can actually cause your string to fray. BCY's wax is designed to penetrate your serving and lubricate the string beneath.

Another good wax is Bohning Xccelerator.

My string snapped three years ago. It had dried out underneath the serving and that caused a weak point. Since then I always wax everything.