Compared to being cooked, are raw white onions good for you? It is true that cooking destroys some antioxidants and therefore, more are bioavailable when the onion is fresh and unprocessed. That being if you want the healthiest option, go for red onions since they have a higher ORAC.
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010