Using the USDA Nutrient Data base http://www.nal.usda.gov/fnic/foodcomp/search/

100 gm Special Dark candy bar - 531 calories
100 gm Milk chocolate - 535 calories
100 gm white chocolate candy bar - 539 calories

However, the darker chocolate has more micronutrients. And I didn't see an option for the darker chocolate up in the 70% cocoa that is supposed to be better for you.

Dark chocolate has the most cocoa solids, and thus the most flavonoids, which are antioxidants and supposedly good for heart health. In addition to having less of the flavonoids, the milk in milk chocolate binds to them, so they aren't as available to you nutritionally. I don't think white chocolate contains any of the flavonoids (just cocoa butter).

If you're going to eat chocolate for health reasons, go for the dark.

"Why is Dark Chocolate Healthy?:
Chocolate is made from plants, which means it contains many of the health benefits of dark vegetables. These benefits are from flavonoids, which act as antioxidants. Antioxidants protect the body from aging caused by free radicals, which can cause damage that leads to heart disease. Dark chocolate contains a large number of antioxidants (nearly 8 times the number found in strawberries). Flavonoids also help relax blood pressure through the production of nitric oxide, and balance certain hormones in the body.

The rest of the article: http://longevity.about.com/od/lifelo.../chocolate.htm

The higher the content of chocolate, the better (but it tastes more bitter the higher you go.)