Due to their mild taste and hint of sweetness without being pungent, sweet onion are the safest choice for people who are not particularly fond of onions. The two most common varieties sold in North America are Vidalia and Walla Walla.Even though their taste may be more universally appealing, their nutrition is not. For those who want to get the maximum amount of antioxidants from the foods they eat, this type is one of the worst choices. Why? Because if you compare the ORAC values of sweet onions vs. red onions, you will find the latter being nearly 150% more. One could argue white and yellow onions are also more nutritious, albeit by a less impressive margin, since their ORAC values are 863 and 913, respectively.
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