If you don't really have the money than do cooking. If you have alot of cash to spare, you can get 99 fletching, 99 smithing, 99 crafting, 99 range (chinning), or 99 herblore.
Many factors need to be considered, including how much you're willing to spend and whether you are botting or training legit.
Otherwise in factoring in it all, Herblore, RuneCrafting, Construction, Fletching, Woodcutting, Prayer and Cooking are suggested by myself as the focused skills to attain level 99 in.
Firemaking is very fast right now with Jadinko firemaking method and the bonfires which were just released,
Fletching would realistically probably take you 2 days from 90-99 if you had the $$ to do broad arrows.
Cooking fast too sharks and then rocks to 99
fletch, hunter, firemaking, mining, summoning (slayer for charms = easy levels) woodcutting, crafting, range. alllevels could be gotten up easy and cheep