I REALLY LOVE CHICAGO...For my age I discovered the pop era first...but by a mile their early work is their best. I like Chicago albums from CTA to VII, but to be honest the only material that has to be considered prog should be their first three albums...and some tracks...for instance 25 or 6 to 4 is clearly a straight rock song...and the suites included in the second and third are very prog for my point of view. Now their big band sound (trombones and trumpets) must be quite annoying for some prog fans. I understand if some prog sites doesn not include Chicago in their lists. Now If the moon has it I don't have any problem...I really dig them
It's a matter of taste