We have always used Budget. We have found they have the best rates, they are prompt at bringing you back to the cruise ship and are there when we get off to bring us back to the rental office.
Just a side note, you may want to drop off your family at the ship rather than go directly to the rental location and then haul the entire family and luggage on the shuttle. We have had them ask some groups to wait until the next shuttle because they would have taken up the entire shuttle and would have put 5 other groups out.