I got a bad car (it died during my trip) from Avis (same parent company as Budget) the one time I used them, and got poor customer service dealing with it as it was going on, so I'm therefore biased against them. It ruined the last day of my vacation. Look on this forum and you'll see a whole thread someone wrote telling people to stay away from Budget.
I've had many good experiences with Enterprise, and always stick with them now. Short lines, good cars, no trying to sneak in extras before you sign on the dotted line, and no false claims about damage to the cars. A few bucks extra up front is worth it.