Of Lucas
Go for what you like the most, trust me, it doesn't feel like much of an achievement if you get it just because of having a 99.
Couldn't have said it better myself =P
Going for a skill that you like is EXTREMELY important, in the long run you will have more fun with the skill, and a less chance to burn out, trust me, it has happened to me ALOT. now with that said, if you just want a cheap and quick 99 then cooking, firemaking, and fletching are your best choices, however i highly recommend something like woodcutting or fishing, as it is quick, easy, and if you do it right, profitable. for example with my 99 fishing it took about a month for 70-91 fishing and another month for 91-99 with a 50m profit from the last 8 levels