I use Hertz pretty exclusively. Its the most expensive of the rental car chains but since I use them for work I can usually get discounted or free rentals through their points. And with the status (similar to airlines) they treat me well -- upgrades, easy in/out, etc. Last time I was there I ordered a mid-size and they double or triple upgraded me to a Caddy. I had to go back to the office to ask them to downgrade me again

For a long time I would use Thrifty in Vegas, didn't have any problems with them. But no matter what agency you select, always join their program. It will save you time and hassles. The other caveat would be to make sure its an on-airport or on-lot. For example, in Vegas if you rent from Fox you take the bus to the rental car center then give them a call and they pick you up from there. That's too much of a hassle.