DLC, preorder cars, gt academy cars, etc would be the rarest. Those aside I don't think there is any car that you cannot get from flipping tickets. Is there?
Edit:If my theory is correct all cars can either be acquired from a ticket or the ncd meaning there isn't really any "rare" cars. Unless you count what your chances are of getting a specific car from a ticket. For example 1 in 64 chance of getting a specific miata from a level 2 ticket. DLC etc aside. (Just an example I'm not sure of the odds or levels of the miatas.)