I rent with Avis as they are a corp partner. Avis rewards points based on dollar spent. Even with PC where I earn 50% more points, I believe I have earned about 5-10 free days, depending on which car class I book.

National works well for those who have short rentals as you earn 1 credit for every 4 days. The free days also do not have any restriction usually as EE, making them much more valuable. Stack that with 1-2 free and you can earn a lot of free days.