My opinion:
F1 - They look silly when multi-billion dollar corporations are unable to beat garagiste teams in preparing these silly-looking little prototype cars that consist of little more than a few bits of carbon fibre glued together. The "hit" to the image is more, as the marques can build whatever they want and are supposedly trying to show their technical excellence... as the money involved is so great, to get beat by a fizzy drink seller and some other privateer teams is not good for the image and the full media and image benefits of participation don't come unless the manufacturer is reguarly winning and in contention for the championship.
Le Mans/Sportcar - Not bad, a good showcase but the profile isn't very high.
GT (Production) - Not bad. Requires a premium sports car in the production vehicle range of course - the cars are the stars, rather than the usually gentlemen racers. Somewhat political with the power/weight formulas, and the profile of this type of racing is pretty low too.
Touring cars - Especially the old super touring style - good exciting bash and crash racing, all volume selling marques welcome. It's not so bad for the image if winning is not acheived. After all, the fans understand that the latest Mondeo has been designed for sufficient rear headroom and not optimal airflow to the rear wing.... Simply participating and being out there is good for the brand, and the involvement definatley promotes a positive image without the "win at all costs" F1 ethos that makes teams look silly when a big car maker can hardly get in the points in a grand prix.
My pick - Touring cars. With a tweaked formula for the world and national championships (WTCC, BTCC, STCC, and bring back the NATCC and Australian 2L series): more exciting and therefore higher profile cars (NGTC with low-cost 350 hp homologated turbo motors, and the classic 19" wheels?), cheap for manufacturers to enter (performance and developments closely controlled - simple, cheap cars), sufficient profile that privateers are able to gather real commericial sponsorship that is sufficient to prepare winning capable cars, it could be the "best" place for of the volume car makers to direct their attention.
If you think manufacturers can promote their wares better in other forms of motorsport apart from circuit racing please select that option (e.g., rallying, drag racing, drifting, solar cars etc....).
