No advice on companies, normall stick with the big ones. Last time national tried to stitch us up for damages, as there was no-one in the office when we returned the car. Make sure you get it from the right terminal, some of the terminals are a ballache as there is no retrun and you have to get a bus between them.
As an aside, Geneva Airport have recent moved their car parkign a bit away from the airport and you need a shuttle bus to go to car and back to terminal when you return it. They don't mention the fact that they add a few euros onto your rental to pay for using the shuttle bus though.
