Insevent is the easiest way to add events to Calendar from your Apple Watch: it is the only application in the App Store specially designed to do this action with a single gesture!
Simply open the watchOS app (included in this Insevent iOS app), pick a calendar, dictate and event and... ¡Voilà! ¡An event will be created in the calendar you have chosen!.
Suppose for example that you are on the street and you remember that you must return a vinyl record to Sam. Open Insevent and dictate: "Return the record to Sam". Voilà! With a single touch in the screen!
Note: the event will be assigned to the current day and assigned the whole day.