Hugely ambitious crowd-sourced racer Project CARS aims not only to look utterly gorgeous but to allow a full career, beginning with karting and ending up in the motorsport of your choice. If it all comes off it could be the definition of definitive. There's more on the development process here and the official site contains plenty of details. For now, here are some screenshots that have been cleverly disguised as photographs of vehicles. Racing game images often convince me that petrol-burning simulators will soon reach the point at which they look more like footage of cars driving fast than actual footage of cars driving fast.
Click to make the cars slightly larger, with a smidgen more detail at the periphery around them. It is only a smidgen but these are the largest versions I have.
Edit: larger versions do exist!