I don't believe it has been mentioned but Terry Kath playing the solo on Chicago's 25 or 6 to 4 is a great one. IF you get a chance watch the recent documentary done by his daughter called The Terry Kath Experience. It was just on TV not long ago, pretty interesting stuff. He was WAY ahead of his time.
First concert I ever went to was Chicago, Syracuse NY, 1970. That was the "good" Chicago before Kath passed away.