When I visited Scotland I hired a car through http://www.bluewheels.com/

Admittedly that was nearly ten years ago, but I found their prices to be very reasonable, and their service to be top notch. I was able to get exactly what I wanted and they handled absolutely every detail for me. They even met me at the train station in Carlisle with the vehicle, despite the fact that my train from London was delayed by over four hours, and I had no cell phone at the time with which to contact them about the delay.

Excellent service on their end, and I've recommended them many times.