The Hardest Skill Cape?

I would count making the cash part of buyables, which still makes them hard for me. But they are not the hardest as making the money is generally fun, it just takes awhile.
The hardest skill for me is Runecrafting, sure you make money but using ZMI you do no profit that much. Any other method is so horribly slow, though ZMI is still slow too.
Agility exp rates are quite good, 50K-70K is nearly as high as woodcutting, around the same as fishing. Sure it can be boring, but the courses are kind of fun, so not as bad to me.