Fletching, but only because it has a higher level than cooking at the moment. If they were at the same level, then it would be cooking for sure. I had 57 cooking and 57 fletching on an account of mine, and I achieved 99 cooking in 1.5 weeks and fletching in 2 weeks. (Playing the same hours of course, and using the fastest methods possible.)
Fletching is alot quicker depending on the bows made.


