What is the best temperature to set the central air conditioning?

72 degrees in summer 75 degrees in winter If you are wanting to save money durning the summer and winter months then set it to

75 degrees in summer 70 degrees in winter
If your goal is strictly for comfort and not energy/environmental concerns, here's what I would go with:

Assuming you have a programmable thermostat, (and why wouldn't you?) I'd set it for 72-74 degrees Fahrenheit while you're at home and awake, 68 degrees while you sleep and 78-80 degrees while you're way. I also recommend you leave the fan turned to "on" (or "circulate" if you have that option) instead of "auto". The blower uses a rather small amount of energy and it'll keep your house more evenly cooled.