It depends what you're doing. If you plan to make a mold after the sculpting you can use oil based modeling clay or WED modelling clay. The best is Monster Clay, it's specifically designed for sculpting portrait sculpture like monster heads, but it's expensive. The green modelling clay from the craft stores (like Hobby Lobby) work pretty well too, just make sure it's sulfur free.
If you don't plan to make a mold, meaning you want the sculpture to dry and use it as the final piece, it's different. I use epoxy clays. The only problem with the epoxy clays is they have a limited working time. They are two parts that you mix to a putty. I've used a number of different ones. I really like Wood Sculpt, I get it off Amazon.
If you can put a little more about what you intend, it will be easier to answer your question.