@Sooner - America's Test Kitchen recently revisited their chocolate test rankings and the top pick was Ghirardelli's 60% Cacao Bittersweet Chocolate Premium Baking Bar, which is also my preference. However, if you're looking for really dark chocolate, I recently tried the Lindt Excellence Bar (70% Cocoa) and loved the results—but that extra cacao makes it extra dark. Good luck!
ETA: Their pick for unsweetened chocolate was Baker's 100% Cacao Unsweetened Chocolate Baking Bar, that grocery store classic. 