I'd go for Monster Clay. When heated up it is very easily malleable, it has the texture of really soft taffy without the stickiness of Chavant for example. It also reacts much better to solvents for smoothing than Chavant or Prima Plastilina.
Here's a video review from someone, kind of gives you an idea what the consistency is:
https://www.youtube.com/watch?v=Od4ySoSFmcg
I heat mine in the oven, by the way.