When it comes to making designer candles, Silicone molds are the best. Due to their flexibility and resistance to heat to a certain extent, you can make very exclusively shaped candles using silicone molds. You can see some of the candles made with silicone molds here, which are never possible using metal molds. The silicone molds need to be sprayed with silicone mold release before casting candles.
The only disadvantage with silicone molds are they have a limited life cycle.