Comet Hale-Bopp was the brightest comet to appear during the 1990s.  It was visible for a very long time (a total of about 18 months), it was very easy to see with the naked eye, and it was widely publicized in the media.  For all of these reasons, it may have been observed by more people than any other comet in history.  

Image details:  A 120-second exposure on Kodak Gold 1000 film, taken with a Canon camera and a telephoto lens at a focal length of 220 mm.