Having rented extensively with all three major US car companies... my generalizations:

Enterprise/National is the best of the three. The cars are usually well taken care of, especially anything in the emerald/executive aisles. The staff is generally good, and I have had zero problems with rental closeouts, false claims of damage, bogus odometers, or an inaccurate fuel gauge with them. All things being equal, I choose EAN if I can, but they eliminated our company's leisure rate so that's meant I've had to choose one of the other two lately.

Hertz... their staff is just mean, off airport or on. I can't phrase it any better than that - I've just had a hard time finding anyone working for them that has any interest in helping their customers. Their cars can be good - and I've scored a number of upgrades with them that I have been happy with over the years. Despite all of the horror stories about bogus reports to the police of car theft, I haven't had any issues with returns. It's almost always at pickup that I have a problem with needing to go to the desk or getting a crabby agent to deal with.

Avis is a bit of a conundrum. I find their off airport locations to be really good - both in the quality of the cars as well as the staff. I've never had a problem at rental or return at off-airport locations from Hawaii to Europe. Their major airport operations, however - are IMO worse than Hertz. Tons of high mileage cars with basic work needed like tires or brakes. They are also understaffed, their systems often break (especially the handheld scanners) - and in the span of a year I've now had three issues with returns not showing up, mileage being wrong - that kind of thing.