September in Algonquin Park
So this year I have already made a two day trek around Algonquin Park and the sights were no less than breath taking. You only need to climb "Lookout" once and you'll understand.
For a brief period the crowds overrun the park as bus loads arrive seeking the colourful sights, but just before and just after the colours peak (usually in prescheduled trips, so their timing is a bit hit and miss). Thats when I go, just before.
Regardless, just go. Just get in your car, pack some fruits and snacks and a good camera, then go.