I generally go thrifty because they are the lowest price (have to use low price for business travel).
CMH - At airport, never had an issue and they usually upgrade
IND - Off terminal. Usually upgrade, but not as fast as CMH
PIT - At airport, never had an issue and they usually upgrade
BWI - Off terminal (all rentals are) - At first they were unsure about processing blue chip, then after taking 5 minutes to review the contract, they gave me the car. A couple of rentals have been like that at BWI. Seems like a training issue.
PHX - They seem friendly and overworked. Contracts/pricing were wrong and took a while to fix. When checking in, they tried to overcharge because they recorded the wrong rental check out time.
SAT - Off terminal, but fast and friendly.
CVG - Off terminal, but very good. Returned later than I planned and they shuttled me right away without waiting for more people.

I have had some difficulty getting rewards credit, but an e-mail to bluechip has always fixed it.