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.
In truth, most people I've ever spoke to use Roma Plastecin/ Plastelina.
I personally can't understand why so many of you use that super-soft Klean-Clay. I used to work in
a bronze foundry and we all used to hate when an artist would bring in their sculptures to be molded and cast.
They were so soft and easily damaged that we had a hard time moving them around to make the molds.
Klean-clay is really only good for use in mold making. I always use it for dividing walls and such.
As far as sculpting goes, I would look up laguna's EM217 (aka WED clay). This stuff is HIGHLY used in the industry (Batman suits) and is absolutely perfect for prop sculpting. It is a special water-based clay that goes up very quickly. I don't even wet it because it doesn't need it. Just cover it in a bag, and it will keep for months. It also holds surface detail very well and only requires a little air from a hairdryer.
The last time they used a WED clay on a bat suit was a while ago - I know I heard reports of a cowl cracking on Batman Forever because and intern forgot to bag the clay - They then went to J525 or some other ridiculously hard clay. If this person is some who's new to sculpting, a wax or oil based clay might be better to learn with no??
Ya, an oil based clay is probably better to learn on, but it is EXTREMELY cheap to practice with WED. The stuff is cheaper than dirt...literally. I got 100lbs shipped to my local clay studio for under $40....My local studio was about 2 hours away, but I live in Iowa so we have little choice on specialty stores.
I can't speak for the Batman Forever incident, but I know from personal experience that WED is the way to go for large projects. I also heard that Cyberman's AMAZING Palpatine sculpt was done in WED. I have one of the castings, and I can tell you that the surface detail is some of the best I have ever seen.
I agree with Kwally89, WED is really easy to use and holds detail very well. I've used it on one bust and currently using it on a second. I had heard about it from Greg Nicotero about 10 years ago, but never tried it out until I also had heard that cyberman used it to sculpt Palpatine. I wish I had tried it years ago, it is great stuff!!!