baseball probably has the single most difficult ELEMENT to it IMO.
BUt, I have wrestled interscholastically, boxed in PAL leagues and played travel soccer, not to mention hoops and hockey in rec leagues.
Hockey, soccer and the combat sports almost definitely require the most endurance in the sense we are talking about. NASCAR definitely has an edurance level to it as well. There are no heat plates making the car ridiculously hot, and you don't have power steering, not to mention the shakes of the car. you inhale fumes and cannot let yourself be distracted for a second, or you could DIE. Not jsut get scored on or kicked in the nuts, but DIE. you need great hand eye coordination to shift into turns etc. nascar or any other auto racing is no joke. ESPN the mag did big spread about 6 issues ago on all that really goes into it, and I was amazed.
As far as wrestling and boxing go, one thing about endurance is that you can't hide within the team like you can with the ball sports.
I remember I was damn in shape in my high school wrestling days, jogging 5 miles a day before school, practice, more running at night. and the fact is, the minute the whistle blew, Iw as winded, its just you and your opponent, both going full tilt. In soccer or hockey (maybe not so much in hockey) hoops etc, you can take a play off if it isn't happening on your end of the field.
Also you have folks chocking you, cutting off your diamphram and punching you in the gut to take your wind away in wrestling and boxing.
Both take a serious amoutn of skill, much like I view baseball. In all three (and nascar IMO) you can't rely on natural gifts, you need the skill set too. Baseball, wrestling, nascar, boxing require the series of moves frequired to be so ingraned you don't have to think about it, becasue it can happen so fast. I know hoops, soccer, hockey all ahve serious elements of it, but in the ones listed, you don't have a chance to catch yourself and recover, you are either pinned, knocked out, crashed or popped out. I respect those four (and golf) the most in terms of skills. Obviosuly golf can't make the cut, because it is almost all skill and none of the other things.
I think at the end of the day, top tier boxing is the hardest. They say the reason the heavy weight division is so bad now is because they all play tight end and power forward in the NFL and NBA now. I know it is a joke, but it has some merit. The effort ain't worth it if you can get it easier than elsewhere.
At the end of the day boxing and nascar offer another element that the other sports don't have to the same degree, death, brain damage, etc.
rugby and football, hockey have it, but it isn't part of the overall purpose of the sport.
I also think amrathon runners hould be high on this list, and maybe tennis players. they are wimps in the "badass" sense, but it is like combinging the endurance of soccer witht he skills of a baseball player.