The Chicago Marathon is one of the most spectator-friendly courses I know. The way in which it weaves back and forth through downtown allows spectators several opportunities to see runners at multiple spots on the course without going too far out of their way.
A spectator can essentially view the course at mile 1, 5k, and Half without venturing more than a few blocks. Being a little more ambitious and creative, a stop at a critical point in Chinatown (mile 21) can be added and you can make sure you’re there to celebrate as your runner makes that last turn into Grant Park for the Finish.
I ran Chicago as my fifth marathon in 2015. I have also run New York City, New Jersey (Long Branch), and Bucks County (Pennsylvania), as well as 40 Half Marathons in three countries and eight States. Chicago stands out because it was the only one where “my fans” (people from New Jersey, no less) where out there cheering in several places. And if you’ve ever run a marathon, you know how important that is.