The Eiffel Power geocache (GC5552H) is the most favorited geocache in France. Hidden in May 2014 by Arnokovic, a volunteer community translator and author of the book 15 parcours pour vivre l'aventure geocaching, it sits within sight of the Eiffel Tower, on the Champ-de-Mars. It was named geocache of the week in July 2017.
What sets it apart is the container and the way you reach it: a small straw hut hanging in a tree, guarded by a butterfly, which you have to climb up and unhook. Hence a terrain rating of 3.5 for a cache hidden in the middle of Paris, and thousands of photos of geocachers perched above the tourists. In his book, Arnokovic mentions maintaining it very regularly and running into the occasional odd encounter, catching geocachers in the act. Climbing a tree at the foot of the Eiffel Tower is surely one of the most unusual experiences geocaching has to offer.
The answer : 3.5
A terrain rating of 3.5 out of 5 in the heart of a capital city is a surprise, and that is the whole point of this cache: the address is ordinary, the approach much less so.