You won't generally find the best in the supermarket--probably either in the US or Europe. There are actually some pretty good more artisanal chocolate bars you can get in many supermarkets to my tastes. But they're still by definition mass market and (often) not as good as what some specialty maker with a small store has.
Ghirardelli are decent, but I've been relatively hard-pressed to find decent American chocolatiers otherwise. In general, European chocolatiers are in a different league.