None of the papers are helpful with F9 – it is a paper on its own 🙂
I would suggest either F4 or F8, simply because F4 and F8 are both written papers, whereas F9 has calculations. It is good to combine a written paper with a calculation paper.
I would not recommend taking F5 and F9 together. Many people will say that they are similar, but they are actually very different and doing both together can cause a lot of confusion.
Depending on where you live, F4 is available as CBE from November 19th. CBE exams can be taken whenever you like, and so what you could consider is taking F8 and F9 in December, and then taking F4 as CBE in January or February.