I used Smooth-on Rebound 25 to make the mold of my scratch build. Its fairly easy to use and yields excellent casts. You can make the jacket for the mold with Smooth-on's plasti-paste or fiberglass.
For casting, you can use either fiberglass or urethane resin. If you use resin, Smooth-on 65D (rotocast resin) is great to slush cast a helmet. This is what the ASOK helmets are made of.
For fiberglass, you will need a gelcoat as the first layer you put in the mold. You can use a polyester based gellcoat , commonly used for boats. It will need to have wax added, or you will have to add a wax additive to cast with it. You can also use a urethane resin to make a gelcoat, like smooth-on Smooth-cast 320. Then you just lay the glass in with regular polyester fiberglass resin. Bondo makes one that is fairly inexpensive and used by alot of prop guys. It has the yellowish/amber color to it. You CAN get a clear polyester fiberglass resin but they are usually a bit more pricey.
This is what i use, or have used, with good results..."the best" can sometimes be a matter of opinion! Anywho...Hope that helped!