This would be up there for sure!
Personally I think the athletes that are in the best physical condition & who train the hardest of all sports are the top MMA fighters! Another very intense sport where top competitors are in unreal physical condition is flat water canoe racing.
I've competed in a number of sports including competitive swim racing, ski racing, powerlifting, bodybuilding, & boxing. I have family members who are cyclists & another who is a triathlete coach (& former competitor) & another who was competing at the World's in cross country skiing. I still say MMA fighting is the toughest with the most gruelling conditioning of all.