Best for what?
The most useful skills are the ones that help you gather resources, and the combat skills. Herblore, too, with which you can create potions that are very useful, but untradeable. But there is really no need to get any skill to level 99.
The most prestigious 99's would be the ones that are hardest to get. Currently, there are less people with level 99 Runecrafting than in any other skill, but this skill is followed closely (in rarity) by Construction, Farming, Crafting, Mining, and Smithing.
In both cases, it is really hard to single out one individual skill. Several skills are very useful, and several skills are hard to get, and therefore prestigious.
The brimhaven Agility Arena.
There is a guy called Lonezer0, he has 99 construction.
The total exp is 13,034,431 experience for a 99 stat on RuneScape.
There is a girl that has 99 on all skills her runescape name is 'xxphatmagexx'.
You must keep hunting. Catching birds or chimpowaws works best.
Power-fishing at Shilo Village is great for fishing experience.
The best place to kill zombies is at the armor zombie place. IT is where everyone get 99 str.
you train it
TRAIN
There are no cheats in RUneScape. Getting level 99 in ANYTHING requires a considerable effort.
you keep training your skill to get up to lvl 99
Only Xidontknow hes lvl 60 and hiz mining is 99 he can mine anything he wants and he gets paid other players ask him to mine for them he is truly the best miner on runescape