I suppose if you have to boil it down (sorry) to a single factor, I'd say the one thing is to ensure the water includes 50ppm of calcium. pH is no use since it may change or not change once you add grains depending upon what other ions are in solution.
The water may be devoid of trace elements needed by the yeast, so use yeast nutrient blend to ensure these are present.
No to be harsh, but taking a fairly complex subject such as water chemistry and asking for a single piece of advice to guide a decision is at best misguided. Without knowing what you stand to gain (and loose!) from switching water, you would be better off sticking with your existing water until you have the knowledge to make an informed decision about why you would want to switch. If you have specific problems with your water, try to address those first so you get some insight into what brewing salts you can add.