I have experience with Europcar, Hertz, Avis and Thrifty.
I used Hertz the most.

Tried Europcar once in a place Hertz was not available. Europcar was fine, I couldn't compare prices but they could not impress me enough to convince me to start renting with them more onften.

Avis: I decided two years ago to give Avis a try as they were a little cheaper than Hertz. They gave me a nice upgrade and the car I received was perfect. What bothered me was the fact you had to pass at the counter. I like Hertz system where you can just go the car get in and drive away.

Thrifty: One experience in Costa Rica. The price was different from the price on my contract. The GPS we received was not working well. They are cheaper but this really was a " if you pay peanuts you get monkeys" experience.

So for me reason number one to stick with Hertz is the ease that your car is waiting and you don't need to get in line at the counter.