ALAMO is the easiest. They have a kiosk for you to check in and it takes less than 5 minutes to complete your transaction. Once your rental agreement prints walk across to the parking garage and pick out a car from the car class you rented - AKA if you rented a mid - size look for the mid-size sign and pick any car you want to drive. All the keys will be in the vehicle. Drive to the exit booth and show you rental papers and ID and off you go. We like Alamo because we get to chose the car we want to drive.
If you don't want to use the kiosk you can go to the counter to do the rental check in but there many be several families ahead of you. The kiosk is so easy and takes less than 5 minutes to use. It very user friendly!!! Or you can do online check in and print your documents at home. Then all you would have to do is go straight to the Alamo location in the garage and pick out your car.
Also if you sign up for the Alamo Insiders program which is the frequent renters program, they offer better discounted rates to their Insider members and the program is free to join and no annual fee either.
** by the way all the major rental car companies are now located right at the airport.