Ok, here is my opinion it really depends on the detail level and size of the part that you are after.
Personally, I prefer a selection of materials:
1.) Waterbased clay for large parts that hold little detail(figs, etc)
2.) Balsafoam sealed with resin for parts with good detail in larger scales.
3.) Super Sculpey for smaller to medium scales (pricey but works well), needs to be baked in order to harden. Fimo clay is a good alternative.
4.) 2-part epoxy putty or similar for small=teenie parts that need to hold good detail, create a rough shape and than remove as needed.
Wire-Loops/Dental tools are a good addition to your tool-chest for any type of sculpting( you can make your own too).
![Wink [;)]](/FSM/CS/emoticons/icon_smile_wink.gif)
HTH.
P.S.: Make your master about 10~12% bigger than needed and emphasize the details.
Won't really matter if you after only one pull of the mold, but casts of any master tend to be smaller and with poorer detail than the master.