The best kind of clay to use for a stop motion clay animation?

One option is Polymer clay. This is a synthetic (man-made) clay that will not dry out for a long time when exposed to open air. However it must be noted that polymer clay can be baked in the oven at home, if desired. The resulting figures will be hardened but often still pliable. You can paint and decorate them. This option makes the clay more useful if you'd otherwise toss it after your project. Other stop-animation artists enjoy using plastalina (also known as plasticine), an oil-based, non-hardening modeling compound. Plastalina contains wax, instead of water, so it will not dry out. A good name to try is Van Aken Clay (plastalina). Both Polymer and Plastacine clay come in a variety of colors and can be found at your local craft store! I sound like an advertiser. Certain Modeling clay will air-dry much quicker than Polymer clay. Dry clay means cracked clay. So read labels thoroughly! Have Fun!!

Plasticine seems to always be the choice of animators.

Plasticine or oil based modeling clay is the normal product. It is now available in many colors and in blocks and sets of small rods of color so you don't have to buy a quarter pound of each to get detailing colors. I normally refer people to Dick Blick just to show an image, but I think the same package is available at Michaels and/or Hobby Lobby with the other clays. Neither tries for a good match between their stores and online sales. Michaels shows only this http://www.michaels.com/art/online/displayProductPage?productNum=kd1127 with Model Magic being an air dry clay. So I end up going back to Dick Blick blocks - http://www.dickblick.com/products/van-aken-plastalina-modeling-clay/ sets - http://www.dickblick.com/products/van-aken-claytoon-sets/ and a bunch more choices http://www.dickblick.com/categories/modelingclays/