Night and day are both best times to visit
In my humble opinion the best time it's morning. In sun day.
During the day the view is amazing and it wasnt very crowded but this was during the week.
I would bet both give you very different experiences. We went early in morning, to avoid the crowd. It was perfect and clear. I would imagine at night, it's mostly lights but you wouldn't see the details of something like Navy Pier
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