Materials wich type of polymer clay is the strongest?

It's not in either of your links, but the clay that I have heard the best things about in terms of strength has been pardo. Supposedly its very flexible, which makes it hard to break. I've heard that someone was able to tie it into a ribbon without it breaking using a very thin sheet. I myself have tried it and it is very nice, very flexible and holds detail well. It's on the more expensive side, but I think it's worth it. You can buy it at most hobby lobbys and there are two different types, art clay and jewelry. I've only tried the art clay so I don't know how the jewelry clay compares to my experience.

Also one more thing I've heard is that it has wax in it so it's more environmentally friendly.

 

Thank you GuroDoLL that was very interesting, I'm glad you took the time to do the research! I think If I had seen your pages before I would probably have chosen to use a mix of Cernit, Premo! and maybe in lesser proportion Sculpey III.
Cernit for it strength after baking, Premo! for his strength after baking (and as to not have it as flexible as only Cernit) and for it's ease for the sculpting part, sculpey III also for making it easier to sculpt before baking. In theory this could be a good combination.

I heard LaDoll clay is very good for sculpting but it's less durable than the oven baked polymer clays, I might be wrong. Since I am not as ambitious (yet) as to cast my dolls to make them out of resin afterwards ,I guess my best choice for my next project would be the mix I described above.

I also want to share some of my personal experience:
My first project, that I'm still working on, is made of Premo!, I find it is a little bit flexible after baking at 130 as recommended, but after several baking sessions it gets harder (less elastic) and I think it also shrinks very slightly.
My doll is 11 cm and it was very difficult to either sculpt the joints before baking or try to carve the holes into joints after baked as it would crack and finally break. So I won't be using any sort of oven baked clay in the future for the joints, or at least for small joints.

 

@ Stinker : You could try baking your joints with the holes already made in the raw clay instead of baking them solid then trying to make holes. Or bake tiny tubes first as a base to sculpt joints (and body pieces?) around those. That's what I have been doing on my 10cm sculpey doll (original sculpey, my guess is it's the weakest of the sculpeys), at least nothing breaks from making the stringing channels.