Most of the big name companies are reliable and have nationwide availability: Hertz, Budget,
Avis, Enterprise, Dollar and National.
Whatever company you are looking at, join their member or free club, this usually offers you discounts. Look around at shopping portals (Southwest, MyPoints, Ebates, Swagbucks, RetailMeNot, etc..) for discount codes.
I just recently learned that Southwest will award Rapid Rewards points for car rentals
retroactively--so if you rent a car you can get RR points after the fact.
Also look at your memberships in travel clubs or with travel credit cards. Your accrued miles or points may buy you a car rental.
Have a fun trip!!