My personal must-have scripts are currently (2025-06-15) these:
youtube-remove-timestamp.user.js: This removes the timestamp from YouTube URLs (clicking on a history entry adds this), after loading. That way, you don't lose your place in a video, when you reload the page for some reason.
youtube-music-high-quality-video.user.js: This forces the quality for Music Videos on YouTube Music to the highest quality available, which is usually 720p.
youtube-embed-quality.user.js: This sets the default quality for YouTube embeds to 4k or maximum available quality.
reddit-side-bar-max-width.user.js: This makes the sidebar be toggable on Reddit, also sets a max width for the sidebar to prevent it from being too wide.
html5-audio-ratio.user.js: This makes the volume slider on HTML5 audio elements use a ratio instead of a linear scale, which allows for more precise volume control. I hate that this is not the default behavior.
The credit for this script goes to the Marco Pfeiffer, I just added a different match URL to it.
half-page-scroll.user.js: This makes Page Up and Page Down scroll half a page instead of a full page, which is useful for reading long articles.
universal-media-hotkeys.user.js: This currently only implements < and > for playback speed on all HTML5 players, similar to how it works on Youtube. Maybe more features in the future.
As a tip, if you want to use one of these without a Userscript Manager, you can make a Bookmarklet out of it and run it on demand. I use this for the audio ratio script on some devices. I tend to use this bookmarklet maker: https://caiorss.github.io/bookmarklet-maker/
Note: This only works if no Userscript-specific functions (GM.*,GM_*) are used.
As Userscript Managers I currently recommend Tampermonkey.
I currently develop on Tampermonkey with Brave. But even with that, there are no guarantees. Websites change, and I don't always notice. Especially for older scripts which I may not use anymore.