The short answer is, you'll get a different answer depending on which forum you're in.
I bowled throughout my teens, been playing pool since I was 15, played organized basketball in my 20s, and trained in boxing, Muai Thai, Jiu Jitsu and Kali in my 30s, and for years I even played Quake 3 Arena online against people all around the world. It's difficult to make a fair comparison among them because they are so diverse.
Some of what I've done are more game than athletics and vice-versa, but you could make a case that all I've listed are sports. All are difficult to become proficient in. Every sport has their Mosconi, Tiger, Michael, Ali, Laver, Gracie, Senna, Kaoklai or Fatal1ty.
But getting cracked ribs or my elbow dislocated in grappling, pummeled, bloodied and knocked down in boxing or Muay Thai and getting bashed by a hard stick in Kali/Escrima and getting bloody hands even while wearing a hockey glove, and almost passing-out three four times from just the training skew my opinion towards the combat sports. During the years I was training in those sports I was always injured, participating in pain, and sometimes not participating because I got injured form the training. It's funny when you get your elbow popped, and the people next to you stop and look at you because it was loud enough for them to hear.
I realize it doesn't make sense to compare the practice and drilling involved with pool or even golf to the drilling, sparring and cardio involved with boxing or MMA. They're two completely different animals. However, in all sports, game or athletics, interactive or solo there is psychology, strategy, difficult training and tactical adaptibility during the heat of competition.
But combat sports also induce an element of fear. Fear or anxiety is something most people experience before engaging in any type of competition. But if you're preparing to step into a ring or octagon knowing you may be seriously injured, in front of a lot of screaming fans that does something to you, in a primal way.
Maybe Daryl Peach knows something about that after his experience in the last WPC, heheh.
I used to work for a guy that was a +1.8 golfer that tied a course record with a 62. He was also a bowler, shot pool and played middle linebacker in high school. We used to discuss this very topic all the time, and it always came down to that each person depending on their background and bias would have a different answer.
I imagine the synchronized swimmers' forum will say their sport is the toughest, despite what Martin Short and Harry Shearer have to say.
