RecentYouTube.user.js

// ==UserScript==
// @name         YouTube - All, Recently Uploaded, Mixes
// @namespace    https://gist.github.com/q00u
// @version      0.3
// @description  Re-order the YouTube button bar so that Recently Uploaded and Mixes are easier to reach
// @author       Phoenix G
// @run-at       document-idle
// @match        https://www.youtube.com/
// @icon         http://f1.allesedv.com/16/youtube.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
//     console.log('Starting YouTube Recently');
    const priority = ['Recently uploaded']; // Array of button titles you want moved to the start
    // This is where we handle the chips array (whenever it loads)
    function handleChips(chips) {
//         console.log('Got chips:', chips);
        const chipschildren = chips.children; // Get all child elements (the buttons themselves)
//         console.log('Got chips children:', chipschildren);
        for (let i = 0; i < chipschildren.length; i++) { // For each child (button)...
            const child = chipschildren[i]; // Set child to this button
            const childTitle = child.firstChild.nextSibling.getAttribute('title'); // Get the title of this button
//             console.log('Looking at', childTitle);
            if (priority.includes(childTitle)) { // Is this title in the priority list?
                chips.insertBefore(child, chips.firstChild.nextSibling.nextSibling); // Move to front
            }
        }
    }
    // Set up mutation observer
    const observer = new MutationObserver(function (mutations, me) {
        // `mutations` is an array of mutations that occurred
        // `me` is the MutationObserver instance
        const chips = document.getElementById('chips');
        if (chips) {
            handleChips(chips);
            me.disconnect(); // I'm done
            return;
        }
    });
    // start observing
    observer.observe(document, {
        childList: true,
        subtree: true,
    });
})();

v 0.3:
Put it in an observer, to wait until the chips array exists