The answer to this question has no 100% "this is it" as there are too many different events that will lock out each other.
Sport Mode usually offers Gr.4 and Gr.3 races, but sometimes leans into Gr.2 and no car belongs to any more than 1 category of them at the same time.
Then there is a difference to be made between road and race cars, which again are polar opposites of each other.
And even asking for the best car of each category again is not outright "this is it" as a car fast on track x may be slow on track y.
You will require a handful of cars to cover a wide range of event types.
But at least for Sport Mode, you can use the "rent car feature" and avoid the cost at the loss of livery options.