This question is part of the best-puzzle award series.
What are your nominations for the best puzzles, here on Puzzling.SE, of the third quarter ( July / August / September) 2021?
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 2021 Q3
- Well received puzzles you may have missed from 2021 Q3
Best by votes/views:
Q: What's the point?
A1: To highlight and encourage good practice in a way that goes beyond upvotes.
A2: 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'.
A3: To prompt members to put forward their own reflections on what makes a high-quality puzzle.