I was in Chicago for a work training this past summer, and I had one of the best burgers I have ever tasted. I met up with a friend who lives there and he insisted we try this place. I believe it was located in the Lincoln Park area, but not 100% sure.
It was at this place called DMK Burger Bar. Grass fed bison patty cooked med, goat cheese, bacon, pickled red onions, and this delicious (but subtly sweet) blueberry BBQ sauce. It was absolutely outstanding and well balanced. They have a great selection of shakes too. The fries were good but not spectacular.