This is my own version of Unity MonoBehavior script Template

1-Scripting__MonoBehaviour Script-NewMonoBehaviourScript.cs

#region Author
/////////////////////////////////////////
//   RAPHAEL DAUMAS --> #SCRIPTNAME#
//   https://headcrab.fr
//   https://github.com/Marsgames
/////////////////////////////////////////
#endregion
using UnityEngine;

public sealed class #SCRIPTNAME# : BaseEntity
{
    #region Variables
    #endregion Variables

    ///////////////////////////////////////////////////////////

    #region Unity's functions
    /// <summary>
    /// Start is called before the first frame update
    /// </summary>
    private void Start()
    {
        CheckIfSetUp();
    }

    /// <summary>
    /// Update is called once per frame
    /// </summary>
    private void Update()
    {

    }
    #endregion Unity's functions

    ///////////////////////////////////////////////////////////

    #region Functions
    #endregion Functions

    ///////////////////////////////////////////////////////////

    #region Accessors
    #endregion Accessors

    ///////////////////////////////////////////////////////////

    #region Utils
    /// <summary>
    /// Checks if all variables are set up correctly, otherwise close Editor
    /// </summary>
    protected virtual void CheckIfSetUp()
    {
#if UNITY_EDITOR
        bool isSetUp = true;

        if (randomVariable == null)
        {
            Debug.LogError($"<b>Random Variable</b> cannot be null in <color=#00f> {name} </color>", gameObject);
            isSetUp = false;
        }

        UnityEditor.EditorApplication.isPlaying = isSetUp;
#endif
    }
    #endregion Utils
}

BaseEntity.cs

#region Author
/////////////////////////////////////////
//   RAPHAEL DAUMAS --> BaseEntity
//   https://headcrab.fr
//   https://github.com/Marsgames
/////////////////////////////////////////
#endregion
using UnityEngine;

public class BaseEntity : MonoBehaviour
{
    #region Variables
    #endregion Variables

    ///////////////////////////////////////////////////////////

    #region Unity's functions
    #endregion Unity's functions

    ///////////////////////////////////////////////////////////

    #region Functions
    #endregion Functions
}

Singleton.cs

#region Author
/////////////////////////////////////////
//   RAPHAEL DAUMAS --> Singleton
//   https://headcrab.fr
//   https://github.com/Marsgames
/////////////////////////////////////////
#endregion

/// <summary>
/// This class is a Singleton, it means that there can be only one instance of it.
/// But it won't destroy the new instance if you try to create a new one, it will just return the existing one.
/// </summary>
/// <typeparam name="T">The type of the class that will be a Singleton</typeparam>
public abstract class StaticInstance<T> : BaseEntity where T : BaseEntity
{
    #region Variables
    public static T Instance { get; private set; }
    #endregion Variables

    ///////////////////////////////////////////////////////////

    #region Unity's functions
    protected virtual void Awake()
    {
        Instance = this as T;
    }

    protected virtual void OnApplicationQuit()
    {
        Instance = null;
        Destroy(gameObject);
    }
    #endregion Unity's functions
}

/// <summary>
/// This class is a Singleton, it means that there can be only one instance of it.
/// </summary>
/// <typeparam name="T">The type of the class that will be a Singleton</typeparam>
public abstract class Singleton<T> : StaticInstance<T> where T : BaseEntity
{
    #region Unity's functions
    protected override void Awake()
    {
        if (Instance != null && Instance != this)
        {
            Destroy(Instance.gameObject);
        }

        base.Awake();
    }
    #endregion Unity's functions
}

/// <summary>
/// This class is a Singleton, it means that there can be only one instance of it.
/// But it won't be destroyed when you change the scene.
/// </summary>
/// <typeparam name="T">The type of the class that will be a Singleton</typeparam>
public abstract class PersistentSingleton<T> : Singleton<T> where T : BaseEntity
{
    #region Unity's functions
    protected override void Awake()
    {
        base.Awake();
        DontDestroyOnLoad(gameObject);
    }
    #endregion Unity's functions
}