Best Food Walking Tour?
A good route is Devon Ave, on the North Side of the city ... from Broadway on the East to Kedzie Ave. on the West. Cuisines: Kosher, Indian, Pakistan, Georgian, Hispanic ... food shops, fruit stores, pastry shops, bakeries, restaurants ... just about anything you'd look for in the way of food. A great route for chowhounds.
This topic discusses some of the available tours: http://chowhound.chow.com/topics/527608
As another alternative, have you considered a "design it yourself tour"? You could choose a neighborhood (such as the ones discussed at http://chowhound.chow.com/topics/437740 ) and design your own tour. I can easily imagine doing this. For example, you could do a "Central Street Evanston food tour" by going to Great Harvest Bread, Foodstuffs, Tag's Bakery, and the Spice House. Or a "Clark Street Andersonville food tour" by starting at M. Henry for breakfast, then hitting Pasticceria Natalina and Swedish Bakery, and if you're not there too early, Hopleaf. Same thing for so many Chicago food neighborhoods - figure out what's in the neighborhood (often along one main street) and walk along hitting the stops you're interested in. Or, you could choose a theme (e.g. pastry shops, chocolatiers, coffee shops, cheese shops, whatever) and find where several are located, and walk around that neighborhood. (I mentioned some of these in the topic on food tours, see link above.)