I usually rent from National....but I do look at all the other companies to see if their rates are better. I usually find that National gives me the best rates in the end.
If you think you'll rent from National, you should join the Emerald Club so you can take advantage of Emerald Aisle. That's when you rent an intermediate and then go to a particular aisle (no need to go to the counter inside the airport) and pick any car in that aisle. You could end up with the intermediate you ordered or you could drive off with an SUV. It depends on what's in the aisle when you arrive.
Mousesavers.com, as somebody has mentioned, has a bunch of coupon and contract codes you can try to find the best rates. They also describe what I just did better. LOL