It's usually a tossup between Alamo and Avis. I avoid the "off brands" like Thrifty because something always seems to go wrong when I rent there. Avis is a shorter bus ride from the terminal than Alamo, but only by 5 minutes or so.

ALWAYS have some kind of discount number to feed to the computer or the reservation agent. These are easy to get and you likely have one from a credit card company or organization you belong to.

The most troublefree and inexpensive arrangement I've found is to get on the Avis "Preferred" list as a frequent renter. You need to rent a half dozen times a year to make the list. You then automatically get the best rates and when you pick up your car, you are dropped off right at the parking spot with the contract inside the car... nothing to sign!