Theres various options. I use Hertz but thats only cuz I have a #1 Gold Membership with them but I have never had any trouble renting their cars. I have seen some good Weekly Rates on their site. (hertz.com). I would personally benchmark the following companies (Hertz, Budget, Enterprise, Avis). Benchmark them by the dates you plan on renting.
NOTE: Airports usually are more expensive to rent cars since there are taxes associated with renting from airports and also ONE WAY (picking from one location and dropping at a different location would cost you more)
I recommend Enterprise. I have done long term rentals of several months with them. Hertz, Avis, etc. will be much more expensive. In the larger cities, there are typically several Enterprise locations. Stay away from airport locations. Check with each Enterprise location as rates can vary a lot from one to another in the same city. You can also negotiate quite a bit on long term rentals. Joining AAA can often save you a lot on your rental.