William - I use Dollar. I've always found them to have the lowest rates. A few tips:

1) Sign up on-line for their Express card before you go. The lines at the centralized rental car pick-up location at McCarron can be a nightmare. You can easily stand in the Dollar line for 45-60 minutes. With the Express card you bypass the line and go straight to the garage. Your name will be on the lightboard and you just walk to your car (keys inside).

2) Check their website often as they change their rates frequently. A rate you may see 4 weeks in advance will drop as your trip gets closer (kind of the opposite of the airlines). I re-booked my 20-day Vegas rental 3 times until I got the car (with taxes/fees) for $429.53.

Enjoy.