I rent cars quite often (not at WDW, we drive up from Miami) and I don't think there is one best company. In fact, I've developed a healthy cynicism toward all the company deals, because I've consistently found
better deals using the power of the Internet.
I'm sure there are numerous other choices, but I usually go to one of the online travel sites (Orbitz, Travelocity, Expedia, etc). I search the city I'm visiting for rates and see what I get.
One thing I've learned is that
none of the sites list by prices as a default -- they list your choices according to which company has paid them how much for that particular listing. So the first thing I do is click on the option to sort by price, from lowest to highest.
Another thing I've learned is to
pay absolutely NO attention to car categories. A particular model of car might be a "compact" with one company and a "mid-size" with another company. So I look for the specific car I want, and then look at price. I also often find that one company's "midsize" car is priced lower than someone else's "compact."
All of the companies offer all sorts of discounts --
AAA, AARP, corporate, senior citizen, etc. I've never seen a single one of those that was lower than the online price.
Once I find what I want, I go to that company's website and see if their own website has a cheaper price than Expedia, et al. I've never found any difference, but I always check. I book either at the online travel site or the rental car company site, whichever seems most convenient (some rental car companies do not require pre-payment, so if there are any questions about cancellation, I book there).
Once I'm booked, I go to the rental car company's site and sign up for their
loyalty program. I've never found any price advantage to those programs, but they're free, and GREATLY improve your pickup time.