I can only tell you the way that we do it, which generally works well:
Use a search engine, like Booking.com or Kayak.
Filter to select one of these companies: Hertz, Dollar or Thrifty (Both owned by Hertz), Avis, Budget or Payless (Both owned by Avis) Stick to these and you'll get better cars and better service.
Filter to make sure you select "in terminal pick up", otherwise you can end up getting bussed to God knows where.
I'd recommend you buy third party excess cover. We have annual European cover for £35, then you can decline all additional cover that they will try to sell you at the counter.
This may not get you the cheapest deal, but we have had some crap over the years from companies like Sixt, and been stuffed for insurance and "damage".
Caveat Emptor, of course.
HTH