Hard to answer that general of a question. The hardest skill is to hit a baseball. I think that's pretty well documented. Physically, however, baseball is not the most demanding sport at all. Hockey or soccer would be. Using high skill levels and physically demanding would be football.
Basketball can be, unless you're in the NBA. Then you coast for three quarters and work the last five minutes of the fourth.
Basketball can be, unless you're in the NBA. Then you coast for three quarters and work the last five minutes of the fourth.