My choices:
1. A good set of sprue clippers
2. A new tube/bottle of eah of the types of glue you use
3. Sanding film/paper in a variety of grits (I love having 200 to 12000)
4. Putty - Any that you want to try out?
5. Fluid Matte & Gloss mediums so you can change how shiney any acrylic paint
6. A quality Resperator/Filter if you're working with toxic stuff
7. A good quality direction lamp with a natural light bulb
8. A nice big self healing cutting mat
9. A selection of different paints you've been meaning to try out
10. Weathering liquids or powders
11. Scribing tools for making your own panel lines
12. Flex-I-File - some people like it a lot
Sorry, couldn't make it to 20.
Nathan