If you were to ask me the best type of clay for sculpting, I wouldn’t be able to answer you without first asking how you’re measuring “best.”

There are clays that will never harden, and they turn to liquid when heated. This is purposeful.

There are clays that air-dry and break down in water.

There are clays that you need to heat in order to harden. Some harden in the oven, others need a kiln and extremely high temperatures.

There are clays that harden upon mixing after a few hours whether you’re ready for them to turn rock-solid or not. 
Each clay is created with a specific purpose and use case in mind. If you need one type of clay, another type might not work at all.

The point is that a sculptor needs to learn to be intentional when choosing a sculpting medium. You know, just like we should be when we’re making any decision at all.