I can't tell you what's the BEST to use, you have to decide that for yourself. Different types of Sculpey have different properties: some are more firm, some softer, some are tacky, some are smooth. Some people prefer one property over another.
I've never used Ultra Light, or Original, but here are my thoughts on the others:
Sculpey III - Too soft for my tastes, so it was real easy to accidentally mess up an hour of work with the slip of a tool or finger. That said, it was a good medium to begin sculpting with, and it's cheap.
Super Sculpey - It's a tad firmer than Sculpey III, but one drawback I didn't like was it is slightly translucent. Seeing the part underneath what I was sculpting made it more difficult to find flaws in my sculpt. Ultimately it was too soft and not as tacky as I like clay to be.
One cool thing is you can boil both of the above in water to harden it, which is helpful when you sculpt onto a plastic piece that can't be baked in an oven.