I’m not sure of the populations, so I’m going with my gut feeling.
Mumbai India had to be the most crowded by far. Fascinating city, but incredibly hot and air conditioning was non-existent except for western-style hotels like the Oberoi.
2nd most crowded, New York. So much to do and so easy to get around.
3rd most crowded, London. A lot to do, easy to get around, and rich culture and history as well.
4th most crowded, Rome. So much beauty there, but not as easy to get around because the subway only has a few lines.