While all chocolate has some health benefits, dark chocolate is generally considered the healthiest of the standard types of chocolate. One reason for this is because of the higher concentration of flavonoids, which provide anti-oxidant benefits. It is also because dark chocolate has less of the unhealthier ingredients often added to chocolate -- less fat and processed sugar.
Dark chocolate is good for your heart because it lowers blood pressure and cholesterol; it also stimulates serotonin, which is a natural anti-depressant. (Please take all health advice given on the web with a bit of skepticism, and as with just about everything, too much of a good thing, even chocolate, is not good for you. Moderation is the key to good health. )