This question is part of the best-puzzle award series.

What are your nominations for the best puzzles, here on Puzzling.SE, of the fourth quarter ( October / November / December ) 2018?

Suggested guidelines for nomination:

• Nominate each individual puzzle in a separate answer, so they can be upvoted/downvoted separately.

• No more than 3 nominations per person.

• Don't nominate your own puzzles.

• Before you nominate a puzzle, check to see if someone else has already nominated it. If they have, then add to that nomination as a comment (or edit it) instead of nominating the same puzzle again.

• In your nomination, explain what it is that (in your opinion) makes the nominated puzzle such a good one.

Some lists to help jog your memory (your nomination doesn't have to be from these lists):

• Algorithmically selected "best of":
• Popular puzzles, both in score and in vote to view ratio, from 2018 Q4
• Well received puzzles you may have missed from 2018 Q4
• Best by votes/views:
• Questions with the most votes from 2018 Q4
• Questions with the most views from 2018 Q4
• Questions with the highest-voted answers from 2018 Q4

_{Meta-meta issues:}

• _{Is this kind of thing allowed on SE?}

  • _{Yes, Photography SE and Sci-fi/Fantasy SE do something very similar.}

• _{What's the point?}

• _{To highlight and encourage good practice in a way that goes beyond upvotes.}

• _{To work towards building a 'hall of fame' of some of the best puzzles on the site (perhaps to reside on a future puzzling.SE blog) - think of it as our 'greatest hits album'.}

• _{To prompt members to put forward their own reflections on what makes a high-quality puzzle.}

This question is part of the best-puzzle award series.

What are your nominations for the best puzzles, here on Puzzling.SE, of the fourth quarter ( October / November / December ) 2018?

Suggested guidelines for nomination:

• Nominate each individual puzzle in a separate answer, so they can be upvoted/downvoted separately.

• No more than 3 nominations per person.

• Don't nominate your own puzzles.

• Before you nominate a puzzle, check to see if someone else has already nominated it. If they have, then add to that nomination as a comment (or edit it) instead of nominating the same puzzle again.

• In your nomination, explain what it is that (in your opinion) makes the nominated puzzle such a good one.

Some lists to help jog your memory (your nomination doesn't have to be from these lists):

• Algorithmically selected "best of":
• Popular puzzles, both in score and in vote to view ratio, from 2018 Q4
• Well received puzzles you may have missed from 2018 Q4
• Best by votes/views:
• Questions with the most votes from 2018 Q4
• Questions with the most views from 2018 Q4
• Questions with the highest-voted answers from 2018 Q4

_{Meta-meta issues:}

• _{Is this kind of thing allowed on SE?}

  • _{Yes, Photography SE and Sci-fi/Fantasy SE do something very similar.}

• _{What's the point?}

• _{To highlight and encourage good practice in a way that goes beyond upvotes.}

• _{To work towards building a 'hall of fame' of some of the best puzzles on the site (perhaps to reside on a future puzzling.SE blog) - think of it as our 'greatest hits album'.}

• _{To prompt members to put forward their own reflections on what makes a high-quality puzzle.}