I usually go with AVIS due to the second driver unless a competitor is so much cheaper that paying them for an additional driver is still cheaper than AVIS...

Having said that with the second driver hiring out of Italy in the last few years despite explaining that I should get a second driver free, and already having put my Exec Club details in the booking to get the second driver free, I still end up being charged for it. They don't understand that at the booking office and I have always been charged. I get the money back but it is a pain having to chase AVIS each time for it.
The only time I didn't get charged was when complaining and getting money back for the previous time they added comments in to my next booking, however the next two times after that I have hired from AVIS there I have again been charged at the time and had to chase for a refund. All of these were booked via BA or part of a BAH booking.