rotation object: Animation VS Skinned Mesh

What is more efficient to animate the rotation of an object or to skin the object and animate the bone ?

So in both cases you are talking about using an AnimationClip to rotate the object. As for the animation itself, it should make no difference whether it is animating a skinned or a non-skinned object.

So the only difference is whether it is faster to use a skinned mesh or a non-skinned mesh, and I'm pretty sure the non-skinned mesh is faster, all other things being equal.

However, things will look different if you use a single skinned mesh to make multiple objects with their own individual rotations. If you need multiple rotated objects and you can make many of them as one skinned mesh instead of as multiple non-skinned meshes, then the single skinned mesh should be faster, since you can have fewer draw calls that way. As far as I know, Zombieville USA used that tricks to have pretty much everything on the screen be a single draw call in total.