There isnt one best resin.
Part of the problem is that resin for casting is resin for casting. SO it very much depends on what you have available where you are.
For instance, here in the UK, smooth cast is available, but I like using a local company instead.
ASsuming you are based in the US - you can give Brian's suggestion a go. IF not, you might want to do what I did, which was call round local companies; speak to the technical speciliasts and tell em what you are up to. They are tend to be really helpful