Without a doubt my favorite is dark chocolate.
Taste is one reason: I do prefer the taste of dark chocolate, although many don't because it is more bitter than the other varieties. (be careful, because some pretenders like Hershey's Special Dark are not really dark chocolate, imho. Although there is no official definition for dark chocolate, to me special dark is just milk chocolate with the milk solids. I prefer a good quality chocolate that is 70% cocoa)
Nutrition is the other reason. Chocolate - nutritious? Are you serious?
Yes. Chocolate has many healthy attributes, such as antioxidants, but those can be negated by the profuse amounts of sugar in milk chocolate and not-so-dark dark chocolates.
White chocolate is not really chocolate: it contains cocoa butter, but no cocoa solids. It does taste good, though.