I believe I choose well SIXT for a car rental company and strongly recommend it. The location at Nimes is quite easy to find (just across and facing the Railway station), the assistant quite helpful and the rented vehicle a brand new one (lucky us).
As a word of advise though when renting, be careful at extra charges. If you like to book in advance and pay for the rent before the service is actually delivered, make sure that charges that you choose to wave (such as auto insurance etc.) are not applied accidentally at pickup time. It happened to us, but the company was kind enough to sort that out after. Cheers for them.