As at the beginning of this year, the most popular brands of cars which are tagged as the most expensive are:
The above stated cars are the known and most common cars which tops the list of most expensive. However, the actual list of the most expensive cars are rarely available to us. This is because certain class of people have made it a habit of purchasing custom made cars which the public isn't aware of. Some of this custom cars are built with specifics, certain expensive specifics which even the known most expensive cars do not have.
Another thing that breaks the list above is the the kind of cars you're referring to. I know other type of cars aside luxury cars which cost way morethan some custom made cars. An example will be duty vehiclesade for the military.
Drawing from the above, it will take morethan the usual research in order to get the actual list of most expensive cars, that's if the information is even available.