I'm a car rental free agent but most of the time I end up getting the best deal via carrentals.com, unless there is some regional player that beats them. I remember using Priceline and Hotwire for rentals a while back, but I find I can always get a better price through carrentals.com... it has literally been years since I've found a better deal through PL/HW.

My approach is usually to get a cancellable reservation through carrentals, then check back and re-reserve as prices come down.