Which ocean has the highest salinity?

The Atlantic Ocean has the highest salinity of the five oceans. There is a distinct decrease of salinity near the equator and at both poles. Near the equator, the tropics receive the most rain on a consistent basis. As a result, the freshwater falling into the ocean helps decrease the salinity of the surface water in that region. At the poles, fresh water from melting ice decreases the surface salinity once again. 

Further Reading:

• Horizontal And Vertical Distribution Of Salinity
•  Salinity Of Ocean Water