In all fairness, let me get very specific as to what make the three fairs special amongst all the fairs that take place in this region.
The Rock Co. 4-H Fair is one of the coolest fairs in the area. There are literally barns after barns of young people who spend the ENTIRE week there with the animals they raise. Not to say that does not happen in some of the more urban counties but it is really cool. There are literally a dozen buildings of various displays of 4-H goods. And they have a sale of animals that is really awesome.
Food wise, a lot of the booths are community groups and they put out some good products. Expect excellent fried cheeses, very good ice cream and a wide variety of Wisconsin foods.
The Boone Co Fair is the one absolutely "can't miss" fair in my house. No matter how we plan, we end up going on Saturday nights when the crowds are really crazy. Saturday night is when they bring out all of the draft horses and do a lot of precision moves. There are a lot of barns, a lot of entertainment but also a lot of commercial booths that reflect the nature and business of the county. They have booths from everything to Green Giant. They also have pig races and tigers and a lot of other things.
And the food. The firemen make home made donuts. The boy scouts and the local dairy both bring ice cream. Roasted corn and pork chops are really great. And another boy scout troop does a baked potato bar. You will not find anything fancy but most of the things are done well. They have some great roasted chicken on Saturdays.
The Sandwich Fair is more of an agricultural fair and is always the LAST fair of the season in this area. Many of my farmer friends who USED to head off to the McHenry Co. Fair have moved to the Sandwich fair as that fair really draws people from the entire region.