I've been using National and have had very positive experience, there's a free loyalty program called the Emerald Club, which does all the usual loyalty stuff, but the big benefit is that with your loyalty membership you can pay for a mid-size car (and use whatever promo, credit card discount, etc) and then select any vehicle from their 'emerald aisle' which is essentially whatever they have that's no pre-reserved. In my experience there's almost always a Charger/Challenger/Mustang to pick, a big SUV, I've seen a couple Audi A4s too. As a car guy it's a much better experience than letting the counter staff shove you into whatever econobox they want to push out, or trying to sweet talk them into something better.