Berries have the most antioxidants second to herbs and spices.  As a group, they average nearly ten times more antioxidants than other fruits and vegetables (and exceed fifty times more than animal-based foods). Colorful fruit are often healthier because they contain the most antioxidants. Berries offer protection against cancer, the liver and brain. It also boosts the immune system. 

Here is a list of berries with their antioxidant content from least to most: 

Strawberries have 310 antioxidant units per cup

Cranberries have 330 antioxidant units per cup

Raspberries have 350 antioxidant units per cup

Blueberries have 380 antioxidant units per cup

Blackberries have 650 antioxidant units per cup and are known to have the highest antioxidant power. 

Source: How Not to Die by Dr. Michael Greger