The model I'm currently working on, with a view to crafting it in paper. This is a lot less simple than Xavier's hoverchair, and I've been on a big learning curve while working on it. The generation 1 Transformers characters lend themselves very well to papercraft, being mostly comprised of straight lines and sharp angles, but even with giant robots there's a degree of complexity that isn't apparent until you try modelling them. Some of the details on this character - his horns and parts of his guns - are easy enough to make in in Blender, but are impractical to make out of paper (unless you make the model really big).
Likewise, with Xavier's hoverchair, my biggest decision has been how to make Xavier, and I've had to settle for making him as a flat cut-out.