In reply to Paul Hy:
We usually book through the 'Do You Spain' website and select 'on airport' and full/full options to see who comes up cheapest etc. This has served us well over the years with Firefly coming out consistently as the best of the bunch in Alicante and Barcelona both for service and cost - prices often around £10-15 for a week in Jan/Feb! Often a queue but get your partner to go ahead and get a ticket whilst you wait for the luggage and it'll work out fine.
Everyone will have had positive and negative experiences with many of the companies listed on the site. (FWIW I won't use Goldcar again).
You can mitigate a lot of the hard sell and hassle at the desk by considering purchasing an annual insurance policy to cover the excess in the event of an accident as others have mentioned. These come in at around £40.
Have used the above site many times as cheap/convenient are my main priorities so if you're the same then I'm sure if you check closer to your dates you'll find a bargain.
Caveat - The 'B' word may change all of the above by the time of your trip. We're heading out in November so guess we'll know more then!