What are the best air dry clays to use?

You're in luck! I've worked with just about them all and here are my test results...

Terra-cotta Clay Bodies

Amaco- Excellent plasticity but a bit sticky. The surface quality is gritty and dries with an uneven skin layer. Its finished durability is brittle and breaks easily. Even though I was careful to score and slip my pieces properly, I still had pieces just fall off when they dried. So frustrating! - $ moderately priced

Activa - Great plasticity. The iron oxide in the clay body stains your hands a bit. Dries evenly to a beautiful terra-cotta rock color. It's very strong and durable. You could bang it on a table top and cause no damage. By far my favorite terra-cotta air dry clay!
- $ moderately priced

Crayola - Good plasticity. The surface quality is smooth but dries unevenly in color. Strong but one drop and it breaks easily. Still comes in a close second to my favorite.

- $ economically priced

White Air Dry Clay Bodies

Crayola - Good plasticity. Dries to a bright white. Can be easily painted with water color, tempera and acrylic paints. Strong, but one drop and it breaks easily. I really like to use this brand because it's easy for kids to use.
$ economically priced

Laguna Dry Hard - Excellent plasticity. Dries with a gritty skin. Hard, but parts fall off easily even if you score and slip them on properly.

Laguna Mexo - Too soft plasticity, very sticky. I didn't like working with it at all! Dries with a gritty skin. Hard, but breaks easily. Some pieces just fell off after they air dried.
$ moderately priced