Who is the current highest ranking military officer?

As of May 2015, Gen Martin Dempsey, U.S. Army, is the Chairman of the Joint Chiefs of Staff, making him the highest-ranking American military officer. His replacement was announced in March 2015, and will be Gen Joseph Dunford, USMC
Admiral Michael Mullen is the current (2010) Chairmen of the Joint Cheifs of Staff making him the highest ranking military officer.