All of the majors are there. I have used Hertz, Avis, Alamo, Thrifty and National. It just depends on who has the best rates when we book. Your travel agent or one of the travel sites like Orbitz can compare rates for you.
If you are just going to one location and don't need a car, then Mears shuttle is convenient and probably the cheapest alternative. If you have three or more in your group consider a town car service. It is door to door with no waiting and with three probably no more costly than Mears shuttle. We use Cartier when we don't need a car.
Yopaulie,
If your a DVC member check with MS, I got a great rate from them with ALAMO, even the Alamo rep at the counter couldn't believe it but I had my confirmation from Disney so they had to honor it.
If your a DVC member check with MS, I got a great rate from them with ALAMO, even the Alamo rep at the counter couldn't believe it but I had my confirmation from Disney so they had to honor it.