There are many sites that you can use to download YouTube videos, but we don't recommend them due to their disadvantages over desktop software. First of all, they're slower due to the limitations of the remote server and your data connection. Many such sites also feature ads of a NSFW variety, or display thumbnails of recently downloaded videos, which may not be appropriate.
Online video converters typically don't give you much (if any) choice about the video file either, in terms of quality or expert format. They only allow you to download a single video at a time, and you can forget about saving whole playlists. 3D and 360-degree videos are also out of the question, and very few can handle 4K.