The best-rated beard trimmer on Amazon.com is the Oster Limited Professional Clipper, as of April 2015. The clipper has a 4.5 star rating with over 1,000 customer reviews.
The Oster Limited Professional works on both hair and beards and comes in five colors, including pink, burgundy, wood grain, chrome and silver. It features a quiet-pivot motor capable of cutting wet or dry hair, fast and accurate haircutting functionality, blade sizes ranging from 000 to 1, and four interchangeable guard combs. It also includes cleaning brushes and oil for regular maintenance. User reviews mention its high reliability, quality construction and its efficacy even when cutting thick beards.