Back awhile there was separation for personal and team sports, so I'll go along that.
For an individual sport, I love playing golf, but I would say singles tennis to be the best there. Its downright exhausting. Running seems too simplistic to be called the best sport. There is a technique to it all, but too much of it is just fitness. In tennis there are all sorts of spins you can put on the ball, placement, finesse, etc.
For team sports, I gotta go with rowing. Soccer is great and all, but you have to be on so much of a higher level of teamwork for rowing. 8 man and 4 man boats are what I row, and you have to be in perfect balance with the other guys on the boat, both in the strength of your pull, and how far you lean to the side. You also have to be perfectly in time with your strokes. All of that on top of the technique! There are no solo moments as in other sports, its just straight up teamwork.