A study published in 2006 ranked 50 most commonly eaten foods in the U.S. by their antioxidant content, and revealed which foods had the highest levels of antioxidant phytonutrients. Here are the findings of the study:

RESULTS:

Large variations in the content of antioxidants were observed in different foods and food categories. The food groups spices and herbs, nuts and seeds, berries, and fruit and vegetables all contained foods with very high antioxidant contents. Most food categories also contained products almost devoid of antioxidants. Of the 50 food products highest in antioxidant concentrations, 13 were spices, 8 were in the fruit and vegetables category, 5 were berries, 5 were chocolate-based, 5 were breakfast cereals, and 4 were nuts or seeds. On the basis of typical serving sizes, blackberries, walnuts, strawberries, artichokes, cranberries, brewed coffee, raspberries, pecans, blueberries, ground cloves, grape juice, and unsweetened baking chocolate were at the top of the ranked list.

CONCLUSION:

This ranked antioxidant food table provides a useful tool for investigations into the possible health benefit of dietary antioxidants.

Research paper details:

Halvorsen BL, Carlsen MH, Phillips KM, Bohn SK, Holte K, Jacobs DR Jr, Blomhoff R. Content of redox-active compounds (ie, antioxidants) in foods consumed in the United States. Am J Clin Nutr. 2006 Jul;84(1):95-135.

This research supports evidence that spices and herbs, nuts and seeds, berries, and fruit and vegetables are all foods with very high antioxidant content, especially berries. However, the data relates to the US and results may not be the same for produce in other countries.

Effects of processing on antioxidant content in foods

The researchers also looked at the effect of processing (peeling, cooking via steaming, boiling) on some of the foods and found that the way a food is processed affects its antioxidant content.

They found that the antioxidant content increased in the following foods when they were cooked (preferred method was steaming): carrots, spinach, mushrooms, asparagus, broccoli, cabbage, red cabbage, green and red peppers, potatoes, and tomatoes.

Steaming was found to be the preferred method of cooking since it clearly resulted in the biggest increase in antioxidant content.

The study found that peeling apples and cucumber decreased their antioxidant content to 33-66% and 50% of the amount in the unpeeled foods.