There isn't a single "highest" college degree. The highest level of academic achievement is generally considered to be a doctoral degree (Ph.D., Ed.D., etc.), but even within doctoral degrees, there's no hierarchy. The value and prestige of a doctoral degree depend heavily on the field of study, the institution granting the degree, and the individual's accomplishments.
Beyond the doctorate, some individuals pursue postdoctoral research positions, which are not degrees but advanced research opportunities. These build upon doctoral work, but aren't themselves degrees.