I usually use Dollar. They let me pick my own car and I have been very happy with the customer service. Note that I joined Dollar Express which is free....not sure if that is a factor. I believe you can sign up for this on their website for free.
The last two times I rented I chose the "lock low and go" rate which was a great deal. With this option you don't choose the class of vehicle, but you still get to choose the specific vehicle within the class they select for you.