www.happylimo.com is a service we used and had good luck with. The cars were clean and drivers professional. Tiffany and FL Tours also receive good reviews here as well.
We've always used Transportation2Disney (I hope this isn't a forbidden one) and never had a problem. They always seem to be somewhat cheaper than the others. They offer a grocery stop if you need it.
Incidentally, on our last trip, we scheduled a town car and found a stretch limo waiting for us when we arrived. It seems the town cars were unavailable, so they sent a limo instead at no extra cost.