If you are buying chicken specifically to make soup, you'll need to look for the best chicken for the job. Ideally, this would be a kosher pullet (a young female that is not quite old enough to lay eggs), one that has been raised in truly free-range fashion eating natural vegetation and insects. It should be about six pounds in weight and freshly killed.
Alas, only those who raise their own chickens can achieve this goal.
The rest of us will have to make do with supermarket offerings. If available in your market, select a soup chicken or fowl. These are older female birds whose egg-laying days are a dim memory. If they are not stocked, they can often be ordered, so check with the butcher. The meat will be tough and stringy, but the stock will be quite flavorful.
More commonly available is the broiler. Opt for the larger size bird which indicates maturity, no less than four pounds. Younger, smaller birds just don't pack the flavor punch. You will probably still need to add canned stock or dry bouillon to boost the flavor.
Cook the whole bird rather than cutting it into pieces. It will take longer to cook and release more flavor into the soup. If you do cut up the chicken, cut at the joints and not through the bone, so that any blood in the marrow won't cloud the soup.