It isn't easy to locate challenge caches in amongst all the other caches, so I recently ran some Pocket Queries and added all the BC challenge caches I could find to a bookmark list, which can be found here
The challenge cache guidelines require new ones to be listed as the Unknown/Puzzle/Mystery ("?") type, but I know of one older challenge cache listed as a Traditional.
I'd like to ask all of you to check out my list to make sure I haven't missed any challenge caches, and also to let me know if you know of any old challenge caches that are listed as a cache type other than the "?" type.
I plan to update my list about once a month or so, but if anyone lets me know about a new challenge cache, I'll certainly add it right away.