Hmmm, this is tough. I think all of them, except the Alley-oop can be done (at least in low frequency, but it could be done) by anyone reasonably coordinated, who practices a lot, while an Alley-oop you need more physical talent to be able to pull it off even once...but you wouldn't necessarily need as much practice.
So really, this all really depends on your definition of "hard".
Anyways, if I had to choose, I would go with hitting a baseball. I think that takes the most work and skill to actually be able to do, since even an incredibly athletic person could not just step up to the plate without ever holding a bat before and do it, while I think someone who is athletic could do most of those other things without a ton of training (OK, the ice skates add a bit of a twist, but I still take baseball).
BTW, I think a much more telling question would be which is the most difficult thing to do CONSISTENTLY. In which case, I would also add in hitting a 15 foot putt.