Inspector dropdown for selecting a tag

Example.cs

using UnityEngine;

namespace derHugo
{
    public class Example : MonoBehaviour
    {
        public Tag Tag;
    }
}

Tag.cs

using System;
using UnityEngine;
using System.Linq;

#if UNITY_EDITOR
using UnityEditor;
#endif

namespace derHugo
{
    [Serializable]
    public struct Tag
    {
        [SerializeField] private string m_Name;

        public Tag(string name)
        {
            m_Name = name;
        }

        public static implicit operator string (Tag tag)
        {
            return tag.m_Name;
        }

#if UNITY_EDITOR
        [CustomPropertyDrawer(typeof(Tag))]
        public class SingleLayerAttributeDrawer : PropertyDrawer
        {
            public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
            {
                using (new EditorGUI.PropertyScope(position, label, property))
                {
                    position = EditorGUI.PrefixLabel(position, label);

                    var nameProperty = property.FindPropertyRelative(nameof(m_Name));

                    var existingTags = UnityEditorInternal.InternalEditorUtility.tags;
                    if (!existingTags.Any(o => o == nameProperty.stringValue))
                    {
                        nameProperty.stringValue = existingTags[0];
                    }

                    nameProperty.stringValue = EditorGUI.TagField(position, nameProperty.stringValue);
                }
            }
        }
#endif
    }
}