Cooking is one of the fastest I personally believe. Fast 99 for sure.

Cooking, firemaking, attack, strength, defense, range, fletching, herb and crafting are fast but expensive

Cooking, fletching, afk prayer and thieving.

They're all pretty easy, just some more time consuming that others.
A but as people have said, fastest in my opinion are woodcutting, fire making, fletching.

Hands down agility is the fastest ;)

I have Fletching, Firemaking, Magic and Thieving...every single one took a little over a week, very easy and would suggest Magic or Thieving because they are slightly more respectable than other ones.

I have 99 cooking and it's literally the easiest and fastest 99 you can get. As long as you have the money you can just buy 1k raw sharks for 850k, cook them, and sell them for 1m. At level 94 and cooking gloves it's around 350k exp an hour. Literally so fast.. Also the best place to cook them is in the basement in the bar in the city Burthorpe. The bank is next to the fire lol.



P.S. This is on OSRS btw lol. I heard RS3 is even faster though.

RS3 is a bit more expensive aswell, if you want a fast skill they don't exist. Most skills take 1-2 weeks if not longer. You gotta find something you like unless you just want the cape for the bonus def and bonus.

I'm personally 400k from 99 wc and its the largest grind of my life.

I recommend fletching since its cheap and fairly quick at about 3 weeks.