IGNguides Ranks Crazy Taxi has a ranking system that scores you on how much money you gather in your run.
Money is, of course, won by taking people where they want to go.
The fares are color-coded. Money can also be gained by performing Crazy Combos while you have your fare. For example, if you pick up a green fare and perform many combos along the way to your destination, you will rake in the cash. The person will be mad, very mad. There are many schools of thought on how to get the best fairs.