The major car rental companies in Antigua and Barbuda include Hertz, Drive-A-Matic, Dollar, Budget, Avis, and several others as listed in the search results. These companies offer a wide selection of rental car options and are conveniently located in various parts of the islands, particularly in popular areas such as St. John's. Visitors to Antigua and Barbuda can choose from a range of vehicles, including compact cars, SUVs, and more, to explore the islands at their own pace.