I'm sure it depends on how much you are willing to spend too. Cooking or fletching is probably one of the fastest though assuming you don't want to waste a stupid amount of gp.
I would say cooking but you lose money for it so i suggest you bot wc to 99 it is the easiest and you get alot of rpofit from it(unless you're powercutting and droping logs)
dungeoneering........... jk construction is technically the fastest if you have tons of money. so is herblore....but if ur not so rich i would go with feltching/cooking/firemaking. keep in mind these are the nooby 99s . u wont get much respect srry to say