Probably Golf with the mental aspect being as big or bigger than the physical side when you're talking about pro golfers. The game is slow so you have a lot of time, too much time, to think about things and put yourself in a bad place.
Auto racing doesn't get the respect it deserves and it's biggest challenge is the high degree of concentration you need, not for moments, or minutes, but hours and if you lose concentration for even a fraction of a second you can die. At the top of the sport they're routinely hitting 4-5 g's many several times per lap -- lap after lap. Although I much prefer F1 to NASCAR the concentration needed for NASCAR is every bit as demanding as F1.
Brian