Cooking is the absolute fastest. It can be done in 48 hours, using sharks and etc. If you're equipping experience enhancers then it can be achieved in way-way less.
Prayer is fast as fuck, as well as summoning.
Thieving and Cooking follow those two.
I'll pitch for Crafting. I know ftp can score 400k / hour with Diamonds. Onyx is faster? Idk.
prayer i don't see how it can be beat. it can be done without cheating in a 13 hour day with no breaks.
Stop saying sht when you don't know what you talking about, doubt you even have 99.

