Guidelines vary, but a good semi-sweet is 58% cacao "content" or less--this is the sum of the cocoa butter and cocoa solids (the dark paste without the cocoa butter)--so the remaining percentage (42) is
essentially sugar, with less than 1% for vanilla and an emulsifier,usually soy-based lecithin. a good chocolate discloses the cacao %--and also has the ingredients on the label. rarely does a manufacturer
disclose more--i.e. the exact percentage of cocoa butter, the exact percentage of cocoa solids.
this is important because two 58% chocolates could behave and taste very differently--even if they are made by the same manufacturer from the same beans--because if one has more cocoa butter, it will be more fluid--and if the other has less cocoa butter but more cocoa solids, it will be more viscous and thick--and taste more bitter in the mouth. but
both would still be listed as 58--still with me?
so a good bittersweet--to follow--is 58% and above cacao content, up to about 70%--though most are in the range of 64-66%; that means the sugar content is 100 minus that %, approximately.
the category above bittersweet is something called "extra bittersweet" and has been all the rage for awhile--anything above 70% cacao--and would have the lowest sugar content of all.
check out this article:
bittersweet that can be bought in stores? my recommendations: try Michel Cluizel--my favorite--from echocolates.com; when I lived in SoHo I saw block SharffenBerger in the Gourmet Garage; and lots of places have block Valrhona now--look for "Manjari" or "Caraibe," my favorite two varieties of Valrhona. NY Cake and Baking has the best price I've seen for block Valrhona retail, though I haven't been there in a while.
you should realize, though, that the whole notion of a good chocolate is relative--it depends on what you are going to do with it. for eating straight out of the hand or for ganache and confectionery, it matters.
for baking--where you are going to heat the chocolate up--it doesn't matter as much and you could buy a less good chocolate and still get good results. but you'll probably find, as I have, that truly excellent
chocolate is addictive--and as hard a habit to break as giving up grinding your own arabica beans and going back to Maxwell House.