You say on 'the' body, which I can only assume means that you're not talking about individuals, but the most ticklish spot for most people in general? In that case, I think it would probably be the soles of the feet, lower stomach, and underarms. However, this isn't the case for everyone, I can say from experience as I have watched many tickling videos that everybody has their own specific 'spot' that's their greatest weakness. If I had to pick one spot, I would say that the feet seem to be a very popular spot, but then it's not often easy to tell who is genuinely ticklish in those videos and who is putting on a show so they can get paid.
Since I'm not entirely sure what you meant, I can say that personally, I'm most sensitive on my upperbody and legs. But again, that's not everyone.
I hope that answers your question, but if I could make a suggestion, you may want to make a poll on the subject or read some polls that others have made. That may give you more accurate answers. 🙂