This one can't be a surprise. Chocolate is to holiday baking as walls are to a building. Always use bittersweet, unsweetened or baker's chocolate for your baking recipes. As a rule of thumb, the sweeter the chocolate that goes into a cookie or pastry, the less you'll be able to taste it when you pull it out of the oven. And since you're doing the baking, you decide how much sugar is going in, anyway. Of course, this rule doesn't apply to recipes such as chocolate chip cookies, in which the chocolate will stay chunky after it's cooked. In recipes that call for cocoa powder or melted baker's chocolate, never substitute chocolate milk powder.
Bricks of unsweetened chocolate are available in most grocery stores and can be chopped or broken according to your needs. Store your chocolate in a cool, dry place, but leave it out of the refrigerator and the freezer unless you're storing it long-term. Chilled and frozen chocolate has the tendency to sweat as it warms up, which keeps it from melting smoothly. Unsweetened chocolate can keep for a couple of years, but milk and white chocolate will only be good for four to six months.
One last tip: When melting chocolate, chop it first. It's easily scorched, and once chocolate burns, it's game over.