Personally I use a cross-platform (Unix at least) tool called YouTube-dl. It's run via the Terminal though, so not for everyone. It works with more than just YouTube though and has many options.
For separate apps, I've only used YTD but it works well enough.
Personally not a big fan of using an extension for it, since it's not something I do that often, and the extension would permanently bloat up my browser then.
Soon, only extensions from either the Mac App Store or the Safari Extension Gallery will work, and seeing that Google doesn't really approve of downloading YT vids, I doubt such an extension will run on Safari in the future anyway