A few years ago, I got a jigsaw puzzle of this monstrosity:
My favourite Escher picture, though, is Reptiles (which, incidentally, also comes in puzzle form) - I'm pretty bad at analysing art and whatever, so I can't really explain why I like it so much - I just do!
1. The obvious link to tessellation with the lizards on the page. So many of Escher's pictures use tessellation very cleverly, and they really enrich a lesson on tessellation and angles - in fact, I enjoy teaching it so much that I'm going to carry on doing so, even though tessellation is no longer technically examined at GCSE.
2. The paperweight is a dodecahedron, one of the five Platonic solids and (using my geeky gamer-type knowledge here) one of only five possible fair dice shapes. If you're going to extend work on tessellation in 2D, what better way than to talk about why there are only five possible completely regular three-dimensional shapes, and I love how this picture ties them together so well.