Scrappy's Bitters, a Seattle company since 2008, has a terrific line up of single-flavour bitters. They use numerous other ingredients to give background depth to their bitters, but it's nice to have have direct uncomplicated flavour names.
Their Lavender and Cardamom bitters are particularly unique and noteworthy. Used extrememly sparingly, they add a serious flavour element to your cocktails. Great in gin- and vodka-based drinks specifically. Scrappy's Aromatic has a lighter touch and a very nutmeg-oriented spiciness compared to other aromatic bitters. Also great are the chocolate and lime for your manhattans, martinis, mojitos and more.