I will echo the words above and say National, hands down. The perks of the Emerald Club are truly unique to the rental car industry, and Hertz and Avis cannot compete (IMO).

Aside from it being a true membership program, its tiered structure allows you to elevate levels based on activity, which reward you with both rental days and/or loyalty points/miles to partner programs. They also run the 1-2-Free Promo every year, which makes it even easier to earn & use free days (which are usuable for super expensive one-ways, unlike the other companies "free" days).