Well this is a tricky question to be answered. It really depends on what it is you're working with how steady your hands are, your tools, the project in its' self...
I personally use Chavant products - They're the best clays out there on the market, the auto makers use their clay for making full-size car and SUV concept applications, and it's the most commonly used clay in the movie biz right now. I, depending on the project will use a Y2Klay, J525, P40, CM70, CM50 or my new favorite, which I am using on the Iron Man, War Machine and Optimus Prime, is the NSP Hard.
The "NSP" means "Non-Sulfur Plasteline" and is great to work with, especially when it comes to your molds - there is nothing in the clay that can prevent the mold from setting up the properly in my experience. You do have to heat it up in a microwave or oven to make it pliable to get it in to the shape you want it, so keep that in mind.
Now, they have an NSP soft that doesn't need a whole lot of heating, if any that I know of, but if your hands aren't that steady, go with a harder clay. Nothing is worse than getting a detail correct and then you go work around it, and the clay moves because it's too soft LoL.
This is a definite question where I have to say, "Help us to help you," in that we'd need more info on your project to help you to the best of our ability for your project.
Hope that helps some what...