I want to answer this in two parts: what I look for in quality belts, and who I think are the best.
#1. Top quality belts have a few key similarities. Most important is the necessity for one piece of supple leather. Cheaper belts are made from thinner pieces of rougher "split" leather glued together to feign quality and thickness. To me, the second key factor is buckle weight. Brass and silver are both nice and heavy. Third, is the leather treatments. The best manufacturers have clever ways of dying, softening, distressing, so create the perfect look - whether you're going for a clean belt to go with slacks or a suit, or a vintage or embellished belt to wear with your jeans.
#2. Who are the best manufacturers? In my view, there are only two belt manufacturers fighting for the right to be called "best." Brave belts from Canada, and the brand I manage, Bill Adler, from Oregon (Bill Adler also makes all of Nike Inc's belts, hence our Oregon headquarters). Brave & Bill Adler both have belts as their core business, which allows focus and attention to detail. The big brands, for which belts are a secondary business, just don't seem to care as much about belts.