The car eligible for the most races would probably be any car made before 1980 that has an mr or fr layout. The best car for this would probably be the 2J or the Toyota 7.
A BMW M5 will win many events quite easy and is allowed to enter alot.
For any event which is reserved for asian manufactures a Nissan R34 is a good choice.
I would also recommend a Camaro for muscle car events it can win lvl 16 historic cup with RaceMod...
The 2J can also be used to grind for credits in the American Championship, which in my book puts it over the top if you are keeping a minimal garage, though you won't be able to use it in Japan or Euro only races obviously.