I was once at an OpenStreetMap conference where 6 out of the 8 talks in one day had an image of the John Snow Cholera Map. And no surprise, it is an excellent, relatable, and interesting early example of GIS. The spatial relationship is unmistakable.
Original map overlaid on modern day London:
The site of the Broad Pump is now the location of a pub called the "John Snow", which is well worth a visit if you are in London.
John Snow location: