ESPN's ratings for most difficult sports, the winner?
 

Here are the top 15 according to ESPN:

1. Boxing
2. Ice Hockey
3. Football
4. Basketball
5. Wrestling
6. Martial Arts
7. Tennis
8. Gymnastic
9. Baseball/Softball
10. Soccer
11. Alpine Skiing
12. Water Polo
13. Rugby
14. Lacrosse
15. Rodeo

The rest could be seen here, including how they calculated the points for the sports:

http://sports.espn.go.com/espn/page2/sportSkills