Historical Autographs can include but not limited to world leaders, military leaders, early flight such as Charles Lindbergh to space flight. However the most popular as mention above in vintage and modern sports autograph with Babe Ruth topping the list. Authors like Charles Dickens are very collectibles especially when found in rare books.
Science and Inventors include famous people in science such as Thomas A. Edison. Political Autographs are very popular also. Some autograph collectors try to get examples of handwriting of all presidents. The easiest to acquire for beginning collectors are the clipped signatures. Clipped signatures are signatures that have been cut off documents or letters. Most serious collectors of today reject these clipped signatures instead look for complete documents or letters.