I don't really think there IS such a discipline.. Each individual horse has it's own limits, whether that be athletic ability, heart, drive.. In different disciplines you want different abilities or traits.. Yes, some of those mix in disciplines that are alike in some ways but I don't think one is better than the other just because of personal opinion. Every one has their preference and what they feel is the hardest on a horse or what takes the most out of a horse.. I think it depends on the horse, not the discipline.