I always use Cicar because it's a local business rather than a multinational. I've used them for at least 15 years and have always found them to be very helpful with my complicated bookings (pick up here, deliver there, in 24 hours, etc). Their cars are well maintained, always clean, and if for some reason they don't have the one you booked you will get a better one at the lower price.

I love the fact that the price you pay is the price you pay - no added extras at the end of the trip, and no additional options (like insurance excess etc) that other companies sting you with at the point of collection. Because there is no excess on damages, you don't have to examine the car for every fault when you collect it (often in the dark, after a long flight).

My one quibble is that they don't fill the tank on delivery, so you have to guess how much you'll use so as not to "give" them petrol at the end.