I needed a car for a little more than a month. I got a Hundai Elantra that averaged 38 mpg (compared to my personal Jeep, that's a sipper!). It was comfortable to sit in for many hours a day, had plenty of pep and a steady cruise control. After 4600 miles, I'm very pleased with my rental.
The first car the agent brought out was a dingy looking car with a noticeably stained interior. I couldn't bear the thought of living with someone else's orange soda and coffee stains for a month, so I asked for a cleaner car. The agent complied.