We have used Tiffany Towncar, but I haven't heard anything bad about any of them.
We liked Tiffany because they don't take the main highway (can't think of the name) into WDW. They take another one that has a higher toll, but is a little faster and less congested. Plus, our driver said they prefer to stop at grocery stores that are off the beaten path. They like this one Kash 'n Karry that is brand new and about halfway between the airport and WDW. We were there on a Saturday morning around 9:30 and it was EMPTY! That is unheard of where I am from. Plus, the prices were decent. Pretty much the same as the grocery stores around where I live. Most of the towncar services stop at the grocery stores in the Crossroads area near WDW. Probably a little more convenient for them, but you are also going to have to pay more. Hope this helps!