There are 3 options, GRP (Fibreglass), Jesmonite and Plaster.
GRP is going to be the best in terms of quality of finish, durability and weight (lightest). Using a GRP gelcoat will give the best quality finish and the hardest surface, so less likely to wear or get damaged. You can also produce the thinnest, lightest mould with GRP.
Jesmonite would be the next best, although not quite to durable and heavier, it is obviously much nicer to use than GRP, as there are no toxic chemicals like GRP.
Plaster is obviously the softest least durable of the 3 however is probably the easiest to use and cheapest. Harder plasters will obviously be more durable.