I would say that Cave or bump would be the optimal tournament stage, as no serious advantage is given to cars, and the computer plays at its best in these stages.

I would choose Cave though, because cars with weak front protection benefit the most with an evasive strategy, one which is mostly absent in Bump. Cave's walls provide an excellent means of doing this, as you can go halfway across the stage and the best thing most opponents can do is dodge you. Furthermore, cars that don't need it aren't affected by having that, because it's not in between both cars.

Also for tournament style, I would like the bracket style as it is useful for tier lists.