I prefer Natinal. With a emerald club card (you could join the club at national's counter for free), you will get 15% discount. For a 2 day weekend, the price for a mid-size car is 60.76 (including tax, no insurance).
If you don't have a gold credit card, buy the insurance. It should be (not sure) about 4/day.
Hertz, national, avis, enterprise, discount, I think those are the big ones you could trust.