Whilst maybe not appearing to be the cheapest at first glance do check out the www.rentacarsalgar.com
I have used the for years and they have always been prompt, reliable and polite.
They do not have a fuel selling policy which I consider a rip off and you do not have the near impossible task of returning the car empty.
There is no extra charge for a second driver and and unlike some other companies there are not complicated additional costs to get an adequate level of insurance.
The cars are well maintained. Do remember by and large you get what you ay for.
The other advantage is that they have an English agent in England who you speak to and the website is translated into good English.
In high season it is necessary to book well in advance.
Poppy Cars also have a good reputation.