We've used Quicksilver Transportation for our last two trips to WDW and they were great. During the summer, my niece sprained her ankle midway through our trip and was forced to spend the rest of the trip hopping in and out of a wheelchair. When they came to pick us up to go back to the airport, our driver saw the wheelchair. When we arrived at the airport, he had us wait while he went inside and brought us back a wheelchair for us to use there.
I've never used Tiffany Towncar but have heard very good things about them as well.
We used Quicksilver because they are a sponsor of this site and the prices are comparable with Tiffany.