This is the premium video download extension. It specializes on YouTube, but works most other locations as well. It allows either the browser or the custom helper to do the actual video download. I believe the reason it is the best is that it uses local resources instead of attempting to oversimplify the interface for the user (requiring remote server, etc.), but there are many reasons it is superior - customizable, merges audio-video streams, extract audio or video only, download multiple videos parallel, etc.