I don't know what to vote for really!
Personally for me, the hardest skill to learn was I suppose the bhs because I was really scared of going backwards but once I had it it was easy - also a simple squat on on bars! I really found that hard! (I was a very low level gymnast, didn't compete or anything!)
Now, to coach, It depends on which level of gymnast I am coaching.
I actually find the more advanced skills easier than some of the easier ones - Cartwheels are really hard to teach to complete beginners!
I'd rather teach a tsuk or yama****a than a handspring vault!
I also find teaching back somersaults hard, to someone who has never done them before!
So overall, like I said at the beginning, I think it is the easy skills which are often the harderst to teach!