I've always been pleased with Dollar and tend to stick with them, I usually end up getting the best rate with them.
It is somewhat a matter of if you have dealt with a rental company before, how you liked them, or if you don't care, keep searching the websites for good deals. I personally don't feel they are as plentiful as pre 9/11, but you can still find deals by checking around and also taking the advice given on these boards.