I am just looking for advice, what is the simplest 3D physics engine that you have used, that works well?

All I need to be able to represent is a spherical ground (planet), one or more tetrahedrons, with 1 or 0 cylinders coming out of each face, which might connect to other tetrahedrons. The tetrahedrons and cylinders have weight and can rotate around their connection point in 1 direction.

Basically, they are simple digital robots that need to walk around, so I don't need something with the complexity of Havok or Newton.