An impartial third party with a high level of experience and expertise in the area of review. Especially good if they can compare the product side-by-side to its competitors.

This can provide its own bias. For instance, your average film critic has seen so many versions of stupid action movie that they find those movies pretty tiresome.

My favorite reviews are currently from America's Test Kitchen. Their equipment reviews are especially insightful and well written to provide a thorough expert opinion on products. Wonderfully, they've found a business model that allows for their independence.