I think Enterprise is the best. If you work for a major corporation check the corporate rate. Some companies have negotiated a reduced (or no) surcharge and that rate may even apply for personal rentals.
If you work for a larger company, definitely talk to your travel department. I was lucky in that I worked for a company that had negotiated a no-surcharge rate with Budget and Avis, so I was able to rent from those companies before I turned 25 without getting gouged.
Some companies demand that under-25 renters buy the collision coverage, which could add as much as $20. to each day. For this reason I suggest that you do NOT use Priceline.
If you go to Travelocity.com and start to book a car reservation, you will see a list of companies, each with a "policies" button you can click and find out their requirements for under-25 renters.