It depends on what sort of thing you want to cast
Glass cast 10 is my preferred for embedded objects and labels etc
Alumilte Clear or Slow for casting blanks which is a Polyurethane.
Polyester is the cheapest, doesn't require a pressure pot but stinks to high heaven and can be quite brittle to turn.
Epoxy and Alumilte require a pressure pot to get the best results and this requires a compressor. Plus molds, mixing cups, stirrers and mica / dyes if you want coloured blanks.
Be wary of Alumilte Amazing clear cast, it's epoxy and not that great, softens with heat like when you sand it