i have used 10mm hdpe which has been pretty good for me. Titanium is also very good, but can be pretty expensive and harder to work with. i like hdpe as it is cheap and very easy to work with, and not that heavy, you can only get it in black or white mostly though, i get all mine from direct plastics.
steel is really heavy so wouldnt bother using that as armour in my opinion. my fw spinner would cause alot more damage to steel than hdpe when i used to test it.