The best you can find.. I use Ghirardelli because it's the best I can get from my grocer. I just made some bittersweet chocolate ganache using Green&Black's organic 70% cacao dark chocolate.. it's yummy, but you have to like bittersweet chocolate. I am using it on homemade eclairs so there plenty of sweet to cut it. I use semi-sweet or a blend of semi and bitter for most applications. It's REALLY good if you dip strawberries in it..
I second the don't overbeat your whipped ganache.. it goes from perfect to a complete mess in no time.