I have also heard the suggestion that 666 is an obvious number to pick if you use Roman numerals, being every letter (less than M) once in descending order: DCL

I have also heard the suggestion that 666 is an obvious number to pick if you use Roman numerals, being every letter (less than M) once in descending order: DCLXVI. (But why exclude M?) (Trivia: what is the biggest number that is an English word when written as a Roman numeral?)