I'll second Bill and Jack's views on Elgin and Onwentsia. In my view, Elgin should definitely be in the top 25, and Onwentsia should be comfortably in the top 5. I've played 13 of Terry's top 25 and 5 of the top 10, and Onwentsia is my favorite course in Chicago. The greens are spectacular, the routing is brilliant (and apparently very close to the original), and there is a ton of strategy. I also love the quiet and beautiful setting; I can't recall a single place on the course where you see (or hear) a house or road. The only thing that could potentially hold its ranking and reputation back is the exclusivity and the fact that raters don't like to 3 putt.
I also think Flossmoor should be rated higher, but I'm biased there (of course, I joined because I love the course and not the other way around). Flossmoor is probably my #2 course in Chicago after Onwentsia. The disparity between my personal ranking and everyone else's suggests that I put a lot more weight on interesting, challenging greens with subtle contours that affect the strategy of the hole. Onwentsia and Flossmoor have those in spades.