Depends what you want her to look like!
I'm a big fan of the Highwaywoman outfit, dyed green, no hat, with the Practical Prince boots. If she's going anywhere fancy, I switch the trousers for the Elegant Princess skirt (trim dyed green to match). She's a redhead so it's sort of a badass Botticelli look.
If you want sexy, the Mercenary outfit's the way forward. I quite like the combo of the Mercenary tights with the Mistpeak jacket & tunic too, but that's just me.
Play around with it. That's the fun of costume changes! That said, there isn't a HUGE variety in Fable 3, so it takes a bit more playing about with than you might expect.