KORD has so many types, we should go by terminals.
On United's terminal you can see many types, all the way from 727s to 747s. United Express has put there few Bae 146s, and new Dornier 328s; you can even spot ATPs.
American has also almost every type they fly - 727, 73NGs, 767s, Fokker 100s, and many Super 80s.
Delta flies 727 shuttle to JFK and 757s to ATL.
On international terminal, you can spot 742s (BA, Virgin, Korean), MD-11 (Swissair), B747 (Lufthansa), A330-300 (Sabena), 767s (SAS, LOT, sometimes Aeroflot), A310 (CSA) etc...
Aircraft operations: average 2490/day
15% air taxi
4% transient general aviation
Chicago Midway - MDW
, has mostly 737s nowdays. It used to have more 727s while ATA & KIWI operated them; regular visitors are also 757s and DC-9 (Northwest) and maybe 717 with the arrival of AirTran on this aiprort. Lots of small turboprops like Jetsreams etc.
Aircraft based on the field: 161
Aircraft operations: average 831/day
Single engine airplanes: 62
Multi engine airplanes: 20
Jet airplanes: 53
32% transient general aviation
% air taxi
<1% local general aviation
Gary Chicago/Indiana GYY
has probably most interesting visitor: new-new Pan Am 727 with winglets
Aircraft based on the field: 77
Aircraft operations: average 169/day
Single engine airplanes: 50
Multi engine airplanes: 15
Jet airplanes: 3
66% transient general aviation
32% local general aviation
1% air taxi
Statistics are from http://www.airnav.com/