[alover]

If you are saving youtube videos, note that older file of a particular clip you saved years ago might be better looking even though the same video is now smaller in size in the same format.

You can always use a few mediaplayer, pause at the same timing to compare picture sharpness. Small details and thin lines should be ideal for comparing.
Check the dynamic range and white balance too.
higher bit rate seems to give better dynamic range, sharpness and white balance.

Because youtube is weird and sometimes encode vp9 in a bigger size, when you see your favourite clip in VP9 in a size larger than H264 MP4 quickly save it. Should be good quality.

H264 cons include blocky pixelated picture when fast forwarding.
For 1080 or more, best to download VP9.