Think about location before anything else. The location of a rental car company should be your first consideration. You want a company that has an office near the place that you will be traveling.
Find out what models each company has in their fleet of cars. Most rental car agencies separate their cars into different classes: economy, intermediate, luxury and full-size vehicles.
Look for a company that offers specials. For example, Enterprise offers 50 percent off of vehicles that are rented during the weekend.
Rent a car from a company that has special features in their vehicles. You may wish to choose a rental car company, like Thrifty, that has GPS installed in many of their cars.
Use an online travel site to compare prices (see Resources below). These websites allow you to compare prices of all rental car agencies found in a specific location.