My experience for the last 5 years or so was that
Alamo was almost always the best rates I could find and I would search pretty extensively.
For some reason for approx the last year it seems that the best rates I am finding are with Dollar.
Those are the two I have used the most, both are equal in my opinion.
I wouldn't hesitate using any of the major companies based on price, I prefer the ones located on site, although I have used SIXT once and they are off site.