David Klhufek

This simple plugin adds useful buttons to the native WordPress HTML editor (Classic Editor).
Currently adds: 
h1, h2, h3, h4, h5, span, hr, nbsp, mox
The “mox” button adds a nice responsive CSS class which you can use to highlight pictures or other elements in your posts.
Note: This plugin works only with the Classic Editor. It does not support the new Gutenberg (Block Editor).
<h3>Compatibility</h3>
<ul>
<li>Requires the Classic Editor to be active. </li>
<li>Designed specifically for the HTML mode of the editor.</li>
</ul>

Quicktags for WP

This simple plugin adds useful buttons to the native WordPress HTML editor.

vancho17

This simple plugin allows you to enhance custom fields with TinyMCE allowing your clients to add rich text content to any area of your site.
Features
<ul>
<li>Automatically upgrades textareas in custom fields to TinyMCE.</li>
</ul>

mceTextarea

This simple plugin allows you to enhance custom fields with TinyMCE allowing your clients to add rich text content to any area of your site.

Quicktags for WP vs mceTextarea