You might want to check out the Transportation Board as they discuss car rental quite often.
We find Dollar to be the cheapest 'at the airport' without using a code. Their rates change often, but it's easy to change your rate if it goes down. Join their club so you can skip waiting in line!
When you check their website plug in your dates, etc. and when it brings up the car selection there should be a 'city specials' button, press that and you get the best rates without using a code.
I presently have a rate of $83.00 a week for an intermediate starting Sept. 20th. Of course with taxes and fees about another 30% is added. Still great.