Theo and Alter Eco are my two current favorites. Chocolove, Divine and Endangered Species aren't bad but but IMO good chocolate, to some people, is like grass fed beef or fine wine, once you've had the best stuff you can't go back
I actually have googled "Best Chocolate Bars" and you can find all sorts of lists. I eliminate any that include chocolate I know I don't care for (like Lindt or Godiva) and try different brands that way.
I've been vegan since 1997, so dark chocolate is almost my only option anyway, and I am thoroughly spoilt by it! The darker it is, the easier it is to feel satisfied by a tiny amount, like 3g. Tesco currently do a lovely Swiss dark chocolate, in attractive red and cream patterned packaging, and used to do a fabulous Peruvian one.
By the way, what's the darkest chocolate people here have tried? I had a bar of the Lindt 99% once, which is meant to be one of the more bitter chocolates in that range. I would nibble small bits with a raisin or two for sweetness, but wasn't really that enthralled. Apparently there are 100% chocolates which are much creamier. The Pralus is meant to be great, but it is so expensive!