Toyota, Mazda, Nissan, Mitsu and Honda are the first to spring to mind. They have cars in all sorts of drivetrain layouts, power ratings, and even rally (or rally capable) cars. I haven't tried Subaru all that much...maybe it can, too.
Opel is one of the few from North City that could concievably compete in just about everything. The from the lowly Corsa to the Calibra Touring Car, the Vectra sedan, the Astra and Tigra rally cars could probably win many races. There aren't enough layout choices, though (just FF and 4WD). I think Ford could stand a chance. Quite a variety, but no wagon if you want to play fair.
I think if you really played by the rules (using a historic car for the historic events; 80's car for 80's events, wagons for the wagon cup, etc.) then you'd probably be forced to stick with Nissan or Toyota.
Muscle Car cup for example, shoots holes in all those theories...the car has to be American if you're going to play fair. (However, Toyota and GM have had joint ventures in the past, so have Mitsubishi and Dodge...hmmm....)
Intersting topic...who's going to give it a shot?