I think National beats Hertz and Avis hands down. Heres my experience:

I rented first few months with Hertz, then National for a few, and then Avis all at the same location (been too long on this project). Always reserved midsize/intermediate.

Hertz was decent. I got good cars and when I did not like the selection, the Gold counter got me something good, unless the rare couple of occasions when there were literally no cars left in the lot. I have gotten Volvos, Mustangs, etc. After being 5*, I had an award rental once at another airport for personal use, and the Gold crew was amazing. I asked if they had any Volvos and they said they dont, but if they did, they would give it to me. They had a brand new Mustang with 2 miles on it, and gave it to me. I have never had any major complaints.

National was awesome. Always a Full Size in the lot, and always something with Sat radio which I prefer rather than a full/mid size. This week, they had 4 cars left in the EC/Exec aisle and none had Sat radio. They still had Camry, Impala, and newer cars with low miles. But I went into the counter and mentioned if they had the keys for a charger parked there with doors locked. They said if the keys are not in, it must be reserved for someone specifically. She asked if I booked a Premium, I said im Exec. I mentioned they dont have anything with Sat radio and asked if I can pick something from Premium, and she said sure.

Then switched to Avis for a month or so to use the 5k bonus mile promo. I got to be Avis First through my company before starting my rentals. Each time, I received an SUV. I have marked in the profile and even complained to Avis who said they have put it in my records not to give me SUV. But still the same goes on. Ok, so they assign me SUVs, fine. I go to the counter and the crew couldnt care less for me. When I insisted I dont want the SUV, they assign me a economy (a downgrade from my res) or the worst midsize on the lot. Actually, once I was assigned an economy before I got there. Plus, there is not automatic door entry (I didnt even know that existed for rental cars). I complained and was given the "there are no other cars left" when I saw multiple Full Sizes and midsizes in the lot and cant imagine every single of those is for an Avis First or higher customer.

I just couldnt be happier going back to National now. The return crew knows me by name and always thanks me and acknowledges my business, while Avis barely avoids a cold shoulder. Hertz return crew is average, no complains. Once the National return lady saw me returning Avis and coming out of the lot, and asked me why I switched. I felt embarrassed a bit and told her it was some promo and I will be back with National.