I have found Alamo or Dollar to consistantly have the lowest prices, Feb prices are not that great right now I'm looking for some for myself, hopefully they will come down some.
But it is always worth it to check them all out, using different combinations of codes, sometimes you can find a deal.
Some here have had good luck with Priceline as well.
mousesavers has a great section for rental car discount codes and coupons. I used one of them listed during my last trip and can recommend those pages highly.
Here's a link to another list of codes. It's time-consuming, but you just have to keep plugging in different codes to see what comes up in terms of rates. I've found the Carlson code generally produces favorable rates.