The best Crossbow in Runescape is the Royal crossbow. It is not tradable, but instead there are 5 peices which you can get from the Grand Exchange or trading with other players. The peices are : Coral crossbow, Royal sight, Royal bolt stabilizer, Royal frame, and Royal torsion spring (the estimated price for all of the peices is around 6 and a half million gold peices). Combine the Royal set with the coral crossbow to get a Royal crossbow (unforged).
Finishing the crossbow. In order to finish creating the crossbow, you will need 70 smithing (note you will need 85 ranged to be able to weild the crossbow). The only thing that will forge the crossbow is the heat from the Queen Black Dragon's fire breath (phase 4). Once you make it to the 4th phase, stand in front of the dragon and use the brandish option. You now have a Royal crossbow!