What chocolate works best depends on the baked good involved. (Ricardo DeAratanha / Los Angeles Times)
Chocolate, much like wine or cheese, can vary vastly from brand to brand, and even within brands. The quality of chocolate depends on the origin of cocoa beans, as well as how they are roasted, processed and blended. And, just like wine, there is no definitive right or wrong as far as taste goes; it is a personal preference.
Professional chocolate tasters break chocolate down by appearance, aroma, break (how it snaps), melt, taste and aftertaste. Depending on the style of baked good you're making, you may value one or more of these qualities more than others. For example, if you're making truffles, appearance and intensity of flavor are paramount; you want a shiny truffle with a rich chocolate flavor. But with brownies, on the other hand, flavor is still important, but appearance less so.
In general, we find that bittersweet and semisweet can be used interchangeably in recipes without altering the outcome. If the chocolate is flat and bland, a chocolate mousse made with it is likely to be bland as well.
Although bittersweet lends an intense chocolate flavor, it often lacks the roundness that works well in chocolate mousse or truffles. It's a good choice for recipes such as bundt cake or souffle, where the chocolate flavor needs to stand up to many other ingredients.
We find semisweet to often have rounder, fruitier qualities that work well in mousses, truffles and any recipe where the outcome depends heavily on the inherent flavor of the chocolate.
Chocolate chips or chunks are best used only as add-ins to recipes; they shouldn't be used for melting or in the place of chopped bittersweet or semisweet chocolate. These chocolates have a different level of cocoa-butter content, as they're designed to hold their shape when baked. They have a different texture in baked goods from melted baking chocolate.