If I were you I`d go for F7, as there are some tax implications in F7 that come handy from F6.
I hope this helps.
It's depend on your preparation
I found F7 worked well with F6.
I would suggest that choose either F7 or F8.
F6 is totally about numbers so if you wish to have some theory go for F8 otherwise If you just want to enjoy your numbers go for F7 (though F7 will have a small portion related to theory as well but it is mostly numbers).
F6 and F8 won't require any assumed knowledge from any previous papers as they will be totally new. F7 however will have assumed knowledge required from your earlier F3 studies.