I always recommend Enterprise Rent-A-Car, and there's a reason for that.
In addition to the fact they are willing to rent to under-25 renters, more than once they've tried to go the extra mile to help me. I also appreciate their hallmark staple of picking me up with the car to take me to the car rental office, which has been helpful in a couple cases. They also seem okay with dropping me off to where I need to be after renting the car. Once I very grateful for this service, as I would have missed my train otherwise!
Enterprise Rent-A-Car is pretty standard when it comes to rates, but they seem to be good about service. (I likewise had good views on Avis).
My only qualm is sometimes the city offices don't have the car that I wanted, or could not accomodate specific car model requests, but that was me being picky.
Overall, I can't see a reason to say something bad about Enterprise after using their service three times now. It always seems like a good choice.
Note: review includes zero rating for Rewards Program. I've never used it, so I cannot comment.