I do a good bit of traveling, and I have fallen into a habit of searching for rental cars on Expedia. With one easy entry (and this works with other travel sites as well), I get a price comparison from a number of different car companies, which I can then sort by price or a number of other criteria.
I have found no real pattern, except that rentals vary widely from city to city. Sometimes rental cars cost a ton, and other times they give them away. I have also found that "discounts" are often not really discounts. I can't tell you the number of times I've seen some kind of discount be WAY more than the cheapest rate.
Another tip is pay no attention to "categories" of cars. For some companies, a Dodge Stratus is a compact, for others the same car is considered a mid-size. Instead, compare car models and prices (one company's price for their "midsize" Stratus may be less than another's price for their "compact" Stratus)
Don't get married to one company. A car is a car, and all of the companies you've heard of are pretty good. That's why I like the searches.
Once you've made a selection, however, stop. Go to that rental car company's website, and take five minutes and register for their frequent renter program (free). Then, go back to Expedia or wherever, place your rental and enter your brand new frequent-renter number.That simple step may save you 45 minutes at a busy destination. You've seen all the hype in the commercials about these programs, but believe me, this is one perk worth signing up for.