Water polo! Is a, very demanding sport!! You have to prepare in all kinds if ways from stength to swimming, to running and especially mentally. Played for 6 years and have coached for 3. Love love this sport!
I m not a pro but it looks like boxing and mma are very challenging.
I believe sailing is the MOST challenging sport of all cause the field is never the same YOU need to think quick and to attend any situation out there (in the sea) without your coach or your teammates. It needs arms, absolutely good abs, balance and strong legs. Its hard but SAILors are the MOST fit athletes . You'll probably won't understand that but if u are curious check "Laser sailing strong winds" that's what I do and find more challenging!