I think the biggest difference in service is whether I'm a tourist with a voucher and if I'm signed up to the loyalty program. Branch managers play a big role, too.
There are many variables, and one person might write off an entire company because of one bad experience at one branch (fair enough, I've done it too!)
Avis I've found great in the UK, not so great anywhere else (mainly Spain, Australia).
Hertz UK: I've rented about 5 times in 5 years from one branch, and had excellent experiences.
Thrifty UK: very picky about damage, but pricing can be very competitive.
The discounts on the flights with car+flight are hard to beat with BA sometimes. I got 4 days, E-Class Merc, vueling outbound, BA inbound to Bilbao for £160 once when the flights alone were about £200.
Because of the good discounts on flights, I would still book BA + Avis. The issues with them in Europe are rarely unique to Avis (i.e. downgrades, vouchers being 2nd class citizens, DCC scam, damage charges etc).