Which metal transfers the most heat?

Hi- I'm doing a green energy project and I'm trying to design a water heater that uses several large bars of metal to gather heat. This heater will be in the tropics, and the temperature will frequently be over 70 degrees Fahrenhiet. Out of all the metal availible industrially or otherwise, which is able to transmit the most heat, uming that it's being heated in bright sunlight at 70 degrees or more?