FastestTube is a Safari YouTube downloader extension that is a great option for anyone looking to download a local copy of their favorite videos from YouTube. FastestTube supports full HD and 60fps downloading, removes ads and annotations from videos, and more. And best of all, FastestTube is also compatible with other popular browsers such as Chrome, Firefox, Opera, and more.
Something to keep in mind: users have recently reported some difficulties with downloading this app for Safari, so you may want to try it out for yourself to see if it works for you. FastestTube should be out with a patch soon if anything.