If you were allowed to install just one YouTube related add-on to your Chrome, YouTube Options could be a good choice. It vastly improves your YouTube experience and hides the annoying things about Youtube like the on-screen annotations.
The list of things it can do is long. You can choose to disable pre-roll and in-video ads, disable auto play, change the default size of the YouTube player, pre-buffer videos, play videos in a loop, and more.
It can also help you manipulate the YouTube website like you can hide the header and footer section, remove comments and related videos, switch to dark mode that dims everything except the video and more.
The unofficial version of YouTube options, available at the developers site as a .crx file, also adds the on-click download option to your YouTube for downloading videos in MP4 or FLV format for offline viewing.