I am a heavy youtube-dl user and in general I would always recommend the newest version, especially on Ubuntu and Debian, not the version from the repositories, so remove it:
sudo apt purge youtube-dl
You have to decide if curl or wget or whatever but I like pip and use it to install the most recent version. But here comes the thing, the pip command is for Python 2 and pip3 for Python 3. As Python 2 is or will be deprecated it is recommended to use pip3 that uses python-pip3 as its backend.
At least you resolved your problem but you can still remove the version of youtube-dl using Python 2:
pip uninstall youtube-dl
On a side note: For security reasons it is better to use pip as your user instead of root.