That depends; I have made many spinnaker socks and here are some considerations: Stiff Mylar material is the best for easy in and out but allows UV degradation if you store the spin in the sock in which case a dark color stiff dacron would be best for easy in and out. Materials made for advertising balloons work very well and are lighter but are not as stiff and will bunch more than the former choices. Nylon will bunch the most. Mesh is a bad choice as it is abrasive and "grabby" and the stiffer a material is the less likely that any water will remain inside the sock while sailing due to the pole angle. Look here for one source of materials and supplies you may need: http://www.sailrite.com/

Good luck!