One of my main interests is fancy cancels on US stamps of the 1800's. If the cancel is on a stamp, the best example is a well-centered complete strike on the stamp (SOTN - Socked On The Nose). If on a cover, it is best to have the cancel "tie" the stamp to the cover. This provides a better basis for the cancel authenticity, as well as identifying the origin and timeframe of the cancel.
