The milk with the most protein is sheep's milk, according to the U.S. Department of Agriculture. Sheep's milk has the highest protein content with 15 grams per one cup (240 ml) of 8-ounce serving. However, sheep's milk is not widely available and has a strong flavor that some people may not like. Among the more common types of milk, cow's milk and soy milk have similar protein content with about 8 grams per cup.