This question is part of the best-puzzle award series.
What are your nominations for the very best puzzle of 2015?
Usual suggested rules as shown below.
Winning puzzle (as judged by total net votes) gets the coveted title "Puzzle Of The Year, 2015"
If you want to get ideas from the quarterly nominations (you don't have to), they're here:
Suggested guidelines for nomination:
Nominate each individual puzzle in a separate answer, so they can be upvoted/downvoted separately.
Not 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.
Is this kind of thing allowed on SE?
What's the point?
To highlight and encourage good practice in a way which 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.