In a medium bowl, whisk together mayonnaise, sour cream, apple cider vinegar, Dijon mustard, yellow mustard, sugar, salt, pepper, and paprika until smooth.
If using, stir in chopped parsley.
Taste and adjust seasoning with more salt, pepper, or vinegar as desired.
Cover and refrigerate for at least 30 minutes to allow flavors to meld before using.
Tips
For a lighter dressing, substitute half the mayonnaise with Greek yogurt.
Store in an airtight container in the refrigerator for up to 1 week.
This dressing also works well as a dip for vegetables or a spread for sandwiches.