The Big Six
People adventuring to Africa in the hopes of going on a safari and seeing wild animals will more than likely be informed about The Big 5. Lions, leopards, elephants, rhinos, and cape buffalo are referred to as such because of the immense skill and difficulty required to track them.
In the Harry Potter collector community, The Big 6 refer to the six most difficult Harry Potter translations to track and obtain. Looking for an Asturian, Greenlandic, Gujarati, Macedonian, Malayalam, or Nepali translation? May the odds be ever in your favor!