From a strictly scientific point of view, a perfect Air/Fuel ratio will be 14.6:1, or 14.6 parts air for 1 part fuel on a gasoline engine. In the real world however, this perfect number changes quite a bit depending on air temperature, engine configuration, and especially on whether your engine is naturally aspirated or turbo/supercharged. Throttle position will change these values as well; with naturally aspirated engines hovering closer to 14.6:1 during idle or light load cruising. Suggested numbers below are what should be found at wide open throttle.
For naturally aspirated engines, 13.0:1 is a relatively safe Air/Fuel ratio. Factory-tuned cars however may run more richly than this at higher RPM to make sure they are as reliable as possible. Using a newer Honda Civic Si as an example, Air/Fuel ratio begins at around 13.0:1 until about 5,000rpm and then drops to a range of around 12.0:1 to compensate for advanced ignition timing, more aggressive valve lift profiles and higher heat. Most factory cars will be somewhere in this range, perhaps a bit leaner on lower compression or older engines.
For turbocharged engines, 12.0:1 is a relatively safe Air/Fuel ratio. Most factory tuned cars will run even richer than this, sometimes in the 11.0 to 11.5:1 range. This is because turbocharged engines can be a lot less forgiving to auto-ignition due to the increased amount of airflow and generally hotter temperatures due to the compressing of intake air. In general, turbocharged engines will run a bit leaner before peak boost pressure is reached to help spool up the turbocharger more quickly, and then increase fuel as boost and engine speed increase. As an example of turbocharged Air/Fuel ratio, the 2003 Mazdaspeed Protege starts as high as 14.0:1 at low rpm before boost rises, and then plummets to an Air/Fuel ratio of 10.5 to 11.0 at peak boost to redline. This is known to be excessively rich and can cause hesitation and throttle response issues. A factory reflash to the fuel mapping resets this closer to 11.5:1 and 12.0:1 which gets rid of many of the problems previously encountered.