“Best” isn’t perhaps the right word. There are bad, poor, ok, good, really good and truly amazing riders in all disciplines. It doesn’t really matter in which one but sometimes - rarely - one sees a combination of horse and rider that take your breath and hold your eye because they are just so beautiful together.
In show jumping, the top riders keep their horses balanced and forward whilst jumping 1.40 upwards, often in a tight space. Moreover, the top horsemen keep their horses sound and sane over months of long journeys, varied going and intense competition. They also have an ability to find and bring on youngsters, right up to the highest level.
In eventing, preparing a horse for top level competition at 4* and 5* involves years of training and development, with the necessary horsemanship to build fitness and stamina whilst carefully educating the horse in three disciplines. There are few eventing riders who have the unique skills to ride at speed for 11 minutes over 40 obstacles when each fence presents a different set of challenges. They have to have precision and control combined with an exuberant courage. Which is why there are few riders or horses able to compete at 5*. Best rider here might be the most effective one. I’ve seen some beautiful riders who never win a thing.
Dressage places perhaps the most emphasis on “best rider” since the horse mirrors the rider so clearly. But even the horse dancing to invisible aids before an adoring crowd of thousands has years of training and, ideally, a varied training regime carefully designed by someone with the interests of the horse in mind. Sometimes that horsemanship is combined with great riding as evidenced at the highest level. It is a different journey but the essentials of riding remain the same.