The best thermostat setting is that temperature at which you feel most comfortable.  However, when extremely high summertime temperatures set in, the best temperature setting is going to be 15 to 16 degrees below the outside temperature.  Remember, your air conditioner can only produce so much cooling, no matter how low you set the thermostat.  And no matter what temperature you choose, once you have found a ‘comfort zone’ leave that thermostat set in one place – turning it up and down constantly only costs you more energy!