Best quality on YouTube

[wodim]

Between these two formats:

43          webm      640x360
18          mp4       640x360     (best)

18 (mp4) is chosen to be the "best". I personally think 43 (webm) is better. In several videos I've tried, it's definitely so.

For example, taking this video as an example, here's a screenshot with 18 (mp4):

And here's another one with 43 (webm):

webm is clearly better (look at the eyebrows for example). I can provide more examples if it's necessary.