The On- Airport companies now include Firefly which has something to do with Ryanair and Hertz, they have the fourth booth which is accessible from the baggage hall as well as the public side, the others are Goldcar, Europcar and Centuaro.
Goldcar is usually busy, I won't use Europcar aagain as I booked a seven seater from there a couple of years ago and they gave me aa 5 seater which was no good as we had a party of 6.
I use Centraro, As a member of their Gold Club they give full/full on the fuel, prioity iin the queue and have a points scheme which give a discount on further bookings. Vehicle wise I did have an issue with the quality of the car on my first hiring from them which they promptly changed and handled another issue involving damage efficiently with no claim against me.
I use a yearly insurance policy from icarhireinsurance.com which covers the tyres/glass etc damage for considerably less than any daily additional charge but a credit card deposit is needed.
I haven't used firefly yet, or any of the other car park pickup companies.
When I researche the 'broker' rental web sites some time ago, Carjet was the only one that gave the full facts about the hire agreements.
Mike