ConsumerAffairs.com classifies Greenfield Puppies as their best-reviewed breeder. Greenfield Puppies are not a single breeder, but rather a business acting to connect purchasers with reputable breeders. Yelp.com is another resource for reviewed businesses, including breeders. With their search engine, you can find highly-rated breeders closest to you. The American Kennel Club -- while not offering specific reviews -- has a selective group of Breeders of Merit. These breeders must meet AKC standards, which may be worth more than online reviews.
Greenfield Puppies stand out even outside of Pennsylvania. According to ConsumerAffairs.com, they are the only breeders with an almost perfect rating. Many dog breeders on the site only have a single star, possibly two. These reviews don't hone in to one particular well-reviewed source.
Yelp.com is a helpful resource but reviews may not be the most credible when searching for a dog breeder. They may be biased or shaped out of popularity rather than integrity.
While the AKC site lacks concrete reviews, they categorize their breeders based on the standards their reputation is built on. According to their site, the Breeders of Merit must "demonstrate a commitment to the AKC community, dedication to their breed(s), and actively promote the sport of purebred dogs." In Pennsylvania, there are eight breeders meeting those qualifications, as of 2015. The AKC site also lists many other breeders that are not in this exclusive category.