Is there any way using foursquare api or some other location based api to get top say 20 checked-in cities in the world? I know we don't check-in to cities but I am trying to get a list of top 20 cities or states in the world which generated most of the check-in data for the day.
No, not with foursquare anyway... although you could make a series of calls based on the lat/long centerpoints of major cities and get top trending venues for each.