The simplest answer to that question is just cacao beans and you will end up with a so called 100% bar. It safe to assume though that most people will not like such a bar, it is an acquired taste, but can be quite good.
For people that like dark chocolate the answer should be cacao beans and sugar, maybe cacao butter, but nothing else. When you look at the ingredients list of larger chocolate makers you will see stuff like vanilla (or even worse, vanillin which is an artificial flavor) or soy lecithin. The reason why vanilla is generally being added is to either mask off-flavors or to guarantee a consistent flavor across batches. That might be fine for mass produced industrial chocolate but I believe a craft chocolate should not do this and rather highlight the actual flavor of the beans. Soy lecithin is an emulsifier added to make it easier to work with the chocolate. In the amounts it is added it doesn't affect the flavor. If you are spending your money on a craft chocolate bar then it should be free of any short-cuts though.
Moving on to milk chocolate. A good milk chocolate should contain just cacao beans, sugar, cacao butter and milk powder. For milk chocolate you must add some cacao butter because the cacao butter naturally contained in the cacao beans is just not enough to make chocolate when adding the other ingredients needed for milk chocolate (that is sugar and milk powder). If you take a look at some mass market milk chocolates you are also likely to see ingredients like butter fat, whey powder or vegetable oil. These are all added because they are cheaper than cacao butter. Some manufacturers also add emulsifiers like PGPR which make it easier to work with the chocolate. PGPR is a substance that has been deemed safe for human consumption in limited amounts. Something I would recommend to just stay away from. Again, not really necessary.
And as always the general rule applies: the better the ingredients, the better the product. Good ingredients are expensive though which is just one reason why craft chocolate bars are a little bit more expensive. I will write another time more about pricing of craft chocolate bars.
At our sales event this coming weekend at the German Holiday Market in Mountain View we will offer only chocolate bars containing cacao beans, organic cane sugar, organic cacao butter and organic whole milk powder. Hope to see you there!