Starting to rethink once I've gotten some I thought would be the ones I wouldn't ever get....now on my mind is Runecrafting, but it'll probably be one of my lasts. :x
99 Slayer or 99 Runecrafting.
Everything else is fairly easy to get including 99 Farming but these two are pain in the ass to get and two of my lowest skills. Even botting it was a nightmare lol.
99 Farming was as first I thought impossible to get till I realized how you are actually suppose to get it. It takes a long time but you only play a few minutes per day so it's actually not as time consuming as one would think.
I think I was 91 farming before I quit, forgot though.