Not really a favourite, this one. But at least it has (something approaching) a Christmas theme.
I speak, of course, of 2014 q.12b, which involved a reflective Christmas bauble and asked students to find an image formed by it, treating it as a convex mirror.
Its quite difficult, with two likely pitfalls for even a well prepared student: the negative values of v and f, and the fact that the focal length is 1/4 the diameter. Many instinctively halved it instead, which in itself is a good example for students as to why they should always draw diagrams.
I'm using it to introduce a (not very festive) Christmas theme to a few classes today and tomorrow. Its available here, if you wish to do the same, under the heading 'Christmas baubles'
Enjoy the holidays!