Agility, fishing, woodcutting, mining and hunting are pretty much the only skills that are useful all the to level 99. Most other skills hit their usefulness peak in the early 90s. I'd suggest agility though, mainly beacuse you get the agile armor on your way to 99, which are very good when training other skills like RC. Also, now when there is advanced agility courses you can get upto 70k exp/h, which is pretty good. Also, the cape looks awesome, always a + Whatever skill you choose, goodluck! :D