Which measure of spread is least affected by the long tail in the graph?

The measure of spread least affected is the interquartile range. The reason is that the interquartile range is the distance from the left side of the box to the right side of the box (the spread of the middle half of the data.

The range would be the distance between both end sides of the box plot and the MAD would also include the data that is at the long tail.