Don't all of the national rental car companies pretty much require a credit card to make a reservation?
Enterprise has been the nation's largest rental car company for several years, although they have historically focused more on the "home town" market (replacement cars for people with their own cars in the shop, primarily) vs. the airport market. They are in the process of buying the parent company of National and
Alamo, however, so they will be much bigger on airports soon.
You'll get mixed reviews on Enterprise. A lot of frequent renters and business travelers don't like them because they're not set up to get you into your car fast and their frequent renter perks aren't as good as those at Hertz,
Avis, etc. People who don't rent all that often, however, sometimes like the extra time they spend getting people into the car, answering questions, etc. In general, they do get high rankings for customer service in various surveys. They've been #1 in the JD Power survey for three straight years, for example.