The great state of Illinois, on August 21, 2017 will see the longest duration of darkness near Carbondale Illinois. SIU will host a viewing event on their campus (I believe at their football field) for the public.
NASA says the eclipse, greatest duration, will be a few miles west of Goreville Illinois. This use to be near Hopkinsville Kentucky, but NASA changed that two years ago. They say greatest duration of darkness will last 2 minutes and 40.2 seconds in the Shawnee National Forest.
Another federal government entity, the United States Naval Observatory (USNO), says the maximum duration of darkness will occur near Makanda Illinois which is a few miles south of Carbondale. USNO says darkness will last 2 minutes and 44 seconds. Which one is right is up for grabs. However, these two points are within twenty miles of each other. Both points are on farmland with one and two lane roads close by. I thing the SIU football option will be a better bet!