This is a free plugin for 720p or lower and does a great job - 5 stars there.

But YouTube has split the audio and video on 1080p now, meaning it has to be processed (joined) before it can be downloaded. Otherwise you get no sound (as with another plugin I tried) Many of the plugins no longer work (not to mention the FFv57 upgrade). Another popular plugin took hours and watermarked the output, then asked for money to fix it. (extortion!)

This one did a much better job, asking for a donation up front for server processing. It was able to process a 2 hr HD video in just a few minutes with excellent results. I happily paid for it.

The only reason it didn't get 5 stars is it uses cookies to upgrade rather than giving you an upgraded plugin. I had to fiddle with my security settings to get it working.