Everyone has their own horror stories and bias. I'd quite happily go Ryan Gosling in "Drive" on one or two staff members of Hertz LHR.

I tend to use Avis more and more these days. It's got harder and harder to wangle good discount codes out of Avis and Hertz these days and the AvisBA price seems to be pretty competitive. Avis preferred is easy to sign up for and after you've shown your credit card the first time it really is quick and easy at the big airport locations. Also, you completely bypass any up selling of insurances and the like at the desk, which I loathe.