I don't really have a fall "go to" fly.
You might be fishing a small brookie stream when the water is low and clear.
Or a limestoner with trout rising for BWOs.
Or a stocked stream that is high and murky after a rain.
You have all different conditions, so the flies you choose will be different.
And that's true all year.
In the examples above, a black wooly bugger would be a good choice for the stream that's high and murky after a rain.
For trout rising to BWOs, use a BWO imitation.
For the freestoner that's low and clear, terrestrial patterns are good.