This is the only add-on that really let's you download YouTube videos, convert them to MP3 and more without ads. Simply works.
And the support is great. Answering in a day.
Update July 2020:
Still the best one :)

Really the simplest way of downloading. A button is added to the YouTube website - no messing with your toolbar or cutting and pasting URLs to separate websites.

As an update - you now get a toolbar option. This is due to YouTube constantly changing their interface. This is a nice option - it means you don't have to wait for a plugin update when this happens - you can just use the toolbar icon for the interim time period.

god

well

wow it's good extension

Well done Akansas

Compact, fast, working YT addon. The dev always updates the addon when it gets outdated.

[FRA] Une extension à la hauteur de mes espérances. Le support est rapide et efficace en cas de souci avec la licence par exemple. Rien à dire, un MUST absolu !
[ENG] An extension at the height of my expectations. The support is fast and effective. Nothing to say, a MUST HAVE !

This add-on works really well, and the video downloads really quickly. I really enjoy using this add-on.