The Hardest Skill Cape?

In the sense of hard skill there really is no hard skill but when it comes to grinding to get the materials or get the skill done i'd have to say summoning. Not only does it cost a lot getting the charms for 99 takes a pretty long time, not only do you have to spend a bunch of time and money to collect the charms you also have to spend money to use the charms to make pouch of course there are ways out of losing lots of money from it but that would just produce more work which makes a simple process not so simple. On another note skills like agility and RC are really not so bad RC is easily 40k+ exp an hr and agility can hit up to 60k+ an hr may require a lot more clicking then most skills but have so little complexity for training them.