Whether you use concrete or asphalt for your driveway or parking areas, there's no simple answer for what thickness is best. The correct answer depends on the weight it will bear, the kind of traffic it will see, the soil conditions, and the subbase of your project.
Think of the surface of your driveway or parking area as the roof of your house. If your foundation is crumbling, the whole house - including the roof - is going to shift. Likewise, if a wall collapses, the roof over that wall is going to sag. Like the roof of your house, the surface of your parking lot or driveway exists to drain off water, snow, dirt and other materials. It protects what is underneath it.
Just like your home props up your roof, your subbase is critically important in the longevity of your driveway and parking area. It must be smooth, level and free of all vegetation. The last thing you want is dandelions growing through the cracks in your driveway!