I planted dahlias for the first time this year and they are stunning. The flowers started blooming in early July and got bigger and bolder each week in October. A hard freeze will zap them in a few days, but I will cut the blooms and bring them inside for another week of show.
The variety I planted is here: http://www.longfield-gardens.com/plantname/Dahlia-Gallery-Pablo
My second favorite fall plant is Brussels sprouts. This crop takes off in late summer and can stand a frost or two. I picked these beauties yesterday.