What is the most vulnerable position to tickle someone in (ie tickles them the most)?

Question

What is the most vulnerable position to tickle someone in (ie tickles them the most)?

Answer 1

Different people feel ticklish in different places. Generally the most ticklish places on the human body are the souls of the feet around the neck and the armpits. Tickle these places repeatedly and the person you tickle will be giggling for hours!

Answer 2

While my personal experience has not suggested that I become much more ticklish in any particular position, I have known a few people who find being tickled while upside down to be considerably more devastating than usual.