If you actually want a skill that doesn't cost money to 99 then I'd say thieving, otherwise fm or fletch. If you have crazy cash Herblore is really fast too.
If you have the money, go with either herblore or prayer. If you need skills that are fast and don't cost too much, go with either cooking or fletching. Good luck!
I'm thinking of fletching coz i'm like 75 atm, But is there anyway of fletching good xp and breaking even? because i know you can fletch mith bolts for like 250k an hour xp, but that way means you lose loads of cash.