Illinois is home to some great golf, unfortunately for the public most of the top courses are private (such as Medinah #3, Chicago Golf Club, and Butler National). However, I have gotten to play a few of the top private courses along with some of the hardest public courses. Here are the rankings of the top 3 I've played:
1. Olympia Fields - North Course. Host of at least 4 major championships in the past (including 2 US Opens and 2 PGA Championships), this 7,200 par 70 course was an absolute bear (especially with the lack of golf game I had in college). Besides the courses length, of course, the bunkers were especially ridiculous - at least 10 feet deep on some holes, they swallow you whole.
2. Flossmoor Country Club - across the street from Olympia Fields, Flossmoor is a course that I get to play often with friends who are members there. Host of a past PGA Championship and several Western Amateurs, Flossmoor is particularly challenging for its greens - they are often small with plenty of undulation, and each one is lightning fast in the summer. Don't be anywhere above the hole on the greens here or it is an almost guaranteed 3-putt for an average golfer.
3. Cog Hill #4 (Dubsdread) - Host of the Western Open and many other prominent golf tournaments, Dubsdread just sounds like a course that will take your lunch, and it does if you aren't careful. With tight fairways that often fall off at the sides into deep rough, keeping it straight is of paramount importance here. The greens (once you get to them) aren't too unfair, though.