Hold your horses because I'm not asking how to get the best resolution with youtube-dl videos, or how to get this or the other quality. That has been asked and answered everywhere. I know. I know. With '-F' you list all available resolutions and codecs, and with '-f' you can choose which one to download.
And yes, with '-f best' you list whatever YouTube has marked with the flag "best" which is usually not the highest resolution. In particular, from 1080p and above the video and audio tracks are stored separately, and the flag "best" is never given to video tracks that do not contain the audio. This means, when videos are available in more than 720p, '-f best' is giving you a low resolution version and you have to list them manually with '-f' and get the highest...
The problem is, YouTube seems to be converting its HD contents to VP9. They also are converting other formats. That is clear when you try to download a video that was uploaded in 2009 where there was no VP9 yet.
I would like to know how to choose the original quality, the video that was uploaded in the first place. If that is not possible because all videos are re-enconed, I would like to know which version has suffered less successive encodings or is closer to the original.
For instance, you may find this now in a video uploaded in 2010:
247 webm 1280x720 720p 1821k , vp9, 25fps, video only, 36.51MiB
136 mp4 1280x720 720p 2324k , avc1.4d401f, 25fps, video only, 42.01MiB
Clearly in 2010 there was no vp9 yet, so format 247 has been re-encoded recently. But 136 is also a recently re-encoded version, because it happens that I already downloaded this very same video in 2010 and I am comparing now, and the file sizes are different!
It is not clear to me then, what YouTube is doing and which version should I choose in order to get the quality that is closer to the original.
I will add that I pay every month to YouTube Red. I know this does not entitle me to download contents, but internet is very slow at home. Long history that doesn't below here. I just wanted to add that at least I pay them a fee for the add revenues I do not produce every time I watch an offline version of their videos.