I travel every 2 weeks... I either use Enterprise or Budget. Budget is the fastest for me, because I have a FastBreak, so I can walk right to my car, without waiting in lines...
It honestly depends on the location. Some places I go, I ALWAYS rent Enterprise, other places I ALWAYS rent Budget... Its a crap shoot really... But if you want a safe bet, usually Enterprise is a good safe bet.
And also... Book a small/midsize car... Then when you get there, say "what do you have for deals on free/cheap upgrades?"
And if you think you are going to use a full tank of gas, PrePay for your gas. I always prepay, that way I don't have to try and find a gas station near the airport (VERY DIFFICULT SOMETIMES)....