As for favorites (or "favourites", if you like), I will go with [this one](https://mathoverflow.net/questions/214728/how-many-rearrangements-must-fail-to-alter-the-value-of-a-sum-before-you-conclud/214779#214779) for narrow selfish reasons. It is his answer to a question of mine that I intended less than fully literally, but he decided to construe it literally.