My 2 cents following Mr. Incredibles lead:
01 Boxing- I did this for about 6 months and competed, it kicked my ass pretty bad, I think I even started a thread about it here. I had to quit due to coaching at the time, but I'm pretty sure I don't have it in me to give it another try not that I have the time again. Requires a bit more athleticism then wrestling, but less skill; they are pretty similar though.
02 Ice Hockey- I've done a lot of ice skating, and I played on a roller hockey team in which you played on rollerblades. Didn't compete at a high level once so ever, so it’s not fair to say, but it wasn't very hard for me, although I could never do it on ice.
03 Football- Played in junior high, I’ve always weighed 20-30 pounds lighter then Ted Ginn except I didn't have his speed, so I couldn't really do the sport very well aka I sucked. So since I couldn't compete very competitively and it was junior high it might not be fair to say, but I thought it was pretty easy. Every wrestler/football player I’ve been associated with which is quite a few have told me they think football is much easier, at the Div 3 college and high school level.
04 Basketball- Wrestling my whole life I was raised to hate the other winter sport known as basketball. So because of that I never developed a true shot. Fast-forward to I’m out of college and I play intramurals as much as possible, I practice more then just about anyone who has no real need to be practicing, my shot has improved to just slightly below average, I'm a competitive by nature person though, so I play defense very hard and hustle more then anyone in the YMCA league, if I could shoot better I wouldn't try to overcompensate with intense hustling, but since my shot isn't there, I usually feel like taking a nap when I get home.
05 Wrestling- Most difficult conditioning of any sport I’ve been in easily, it was also the sport that required me to watch my eating more then any other. The sport takes a lot of you, but is great in the eyes of someone like me who is less than 150 pounds because the small guy can still compete. Injuries are almost 2nd to none.
06 Martial Arts- I did BJJ, never competed, but it was enjoyable to me and I never went home tired.
07 Tennis- Played in high school on varsity, by the end of the game I would usually be huffing and puffing pretty hard, which always got me a lot of crap since I also run cross country and wrestling. Tennis can be pretty draining if you have the desire to win (but if you have the desire to win anything will become difficult).
08 Gymnastics- Never tried
09 Baseball/Softball- Played until 8th grade, does not take an exceptional athlete. I've never had to stand in a box though and face an 80 mph curveball; I can't imagine that being easy, requires amazing hand eye coordination at the top level of play, but a pretty easy sport in high school for sure.
10 Soccer- Played for a long time as a kid. Any athlete can become a pretty good player in a short amount of time. Demanding (conditioning), but not demanding of skill. well said)
11 Skiing: I go water skiing often not too bad, alpine skiing I’ve never tried.
11 Water Polo- I'm not a good swimmer, never tried.
13 Rugby- Never tried, I’m too small, my roommate plays this and cornerback for football, he thinks they are pretty equal. But my roommate is the rugby player basically used exclusively for his speed so he might not be the right person to be asking.
14 Lacrosse- Never
15 Rodeo: Steer Wrestling- Never
16 Track and Field: Pole Vault- Never
17 Field Hockey- Never
17 Speed Skating- Way overrated in my opinion, much easier then the 'sport' below it. If you can sprint, you can be taught to skate.
19 Figure Skating- Very difficult it is a bit like gymnastics on ice.
20 Cycling: Distance- Endurance required, but this is an activity that people often pick up in their late 30's thus not that difficult. If you take it to the pro level then yes, but any sport is difficult at the pro level.
20 Volleyball- If you can jump, have height, and are capable of being aware and alert, pretty easy sport IMO