It's time for a subjective quiz. Think hard now. Because Mount Whitney, Mount Rainier, and Mount Elbert actually belong to all three lists, they contribute nothing to the comparison and were therefore omitted.
Several definitions are in use.
The height of a peak above the highest saddle connecting it to a higher peak. The height of a peak above the highest contour encircling it and no higher summit. If not, think of it as follows. As the water continues to drop, the island grows larger and merges with other islands. For a while, Peak X will be the highest point on its island.