It's amazing how simple the answer is and yet the Watchtower organization seems completely oblivious to it. Revelation itself tells you what the number is: "a man's number", "the number of his name". The writer of Revelation spells it out and yet they don't see it! Not only that, the answer is not really a mystery at all. It has been known for centuries and is even corroborated by textual variations of Revelation! See the video below:
https://www.youtube.com/watch?v=UkZqFtYtqaI
I think this is the most reasonable and historically sound explanation I have ever heard for the meaning of 666.