In a medium bowl, whisk together mayonnaise, sour cream, and buttermilk until smooth. Stir in parsley, dill, vinegar, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Blend well and adjust seasonings, to taste.
Cover and refrigerate for ideally 8 hours (4 hours at a minimum) or overnight. If the consistency is too thick after chilling, thin out the ranch dressing with a splash of additional buttermilk.
