Dark Chocolate is one of the best sources of Resveratrol, polyphenols and Antioxidants. It also has the highest ORAC antioxidant score per ounce (6,000 per oz) along with Blueberries. As a comparison, one Vindure 900 tablet has an ORAC value of 19,638

Scientists have discovered powerful polyphenols in cocoa, and recently identified trans-resveratrol in dark chocolate and cocoa liquor extracts, which also contain Catechins similar to those found in green tea and procyanidins found in Red Wine. There have been 100s of human clinical trials using dark chocolate as a source of FLAVANOLS, a kind of polyphenols in wine and green tea. 

The best dark Chocolate MUST have a high cocoa value (70 – 85%). Often flavanols are removed from dark chocolate to reduce the bitter taste. Milk chocolate is generally about 30% cocoa, and has added sugar (with calories) and milk fats, and much of the polyphenol-flavanols have been removed. The best dark chocolate has minimal sweeteners and sugars. Of course, the amount of dark chocolate eaten daily should be kept relatively small due to caloric intake —  a daily dosage of about 1 oz containing about 50 calories is recommended.