I generally rent from Enterprise and have been satisfied with their service so did the same on this trip to Phoenix. Enterprise is located in the communal Rent-A-Car center of Phoenix Sky Harbor Airport which is almost 3 miles away from the main terminal building and takes about 20-25 minutes to get to considering the wait for the shuttle, loading, drive and uploading time.
Service is great you check-in on the main level and get your paperwork processed, then proceed to the lower level where you are assigned a car. On Tuesday evening when it was my turn to pick up a car they were very busy and I had almost a 20 minute wait before I got my car and drove off. Return is also equally painless and hassle free. Service experienced here is so good that this location has become the bar that other Enterprise locations should aspire to beat (they have narrowly edged out the service at Enterprise Rent-A-Car at the Kansas City Airport location).
A small tip for those who choose to refuel their cars before returning them. There are no gas stations around the Rent-A-Car Center so the nearest one that I found was a Circle K at 1602 E Washington St which is accessible via exit 148 from I-10 which is the same exit you take to reach the Rent-A-Car center (go left on Washington for about a mile and it will be on the right hand side).