well in my opinion technically you can get any skill 99 as long as your smart about botting,
however I would assume fletching would be the quickest and lesser obvious one to bot,
but before you go buying anything wait for other people to suggest, I have ZERO experiance in botting, last time i used a 'bot' was a stupid mouse recorder back in 2005 lol
If you do go with a RC bot, have fun waiting a year to get 99.
I'd suggest botting WC and doing Ivies as soon as you can.
You can get 99wc extremely fast from Ivy. I have gotten 99 wc TWICE at Ivies with ZERO botting at all, so I imagine botting would make it all the more easier.