Selection.activeTransform

Description

Returns the active transform. (The one shown in the inspector).

This will never return prefabs or non-modifiable objects.

using UnityEngine;
using UnityEditor;

class Example : EditorWindow
{
    class SelectionActiveTransform : ScriptableObject
    {
        [MenuItem("Example/Duplicate at Origin _d")]
        static void DuplicateSelected()
        {
            Instantiate(Selection.activeTransform, Vector3.zero, Quaternion.identity);
        }
    }

    //The menu item will be disabled if nothing, is selected.
    [MenuItem("Example/Duplicate at Origin _d", true)]
    static bool ValidateSelection()
    {
        return Selection.activeTransform != null;
    }
}