Which electrolyte is most important for century rides? Besides keeping up with hydration and carbs, which electrolyte is most important in maintaining muscle endurance for longer rides like centuries?
I'm guessing it would be potassium or calcium. 
Trying to plan ahead on which snacks or drinks to use for my next ride, RSVP, Seattle to Vancouver.
Most likely sodium. 

However, a lot of it depends on the person. One person might need more of one electrolyte than another, so it's difficult to make a judgment on the order of your "most critical" electrolytes. A sweat test would be a great way of testing which it might be.