A few of my favorites: Polyform clays & Magic Sculpt
Model Air and Sculpey are both polyform clays. The difference is that Model Air doesn't require baking. It dries in about 24 hours and doesn't require baking. Whereas Sculpey will not harden until it is baked. Both are non-toxic, can be painted with latex paint, and are easy to find. The only downside is durability.
Magic Sculpt is a two-part epoxy clay. The only downside to this particular clay is that it is expensive. That being said, Magic Sculpt is my favorite clay to work with. I use it for all sorts of things (presentation models, accessories, construction repairs, architectural elements, swords, replicating lost board game pieces...I could go on). Seriously, the potential uses for this clay are absolutely endless and I LOVE, LOVE, LOVE it.