Here's a good explanation of YouTube thumbnails:
https://www.sitepoint.com/youtube-video-thumbnail-urls/

I'm not sure personally why you'd want any of the inferior quality ones though, especially because in Composr templates will increasingly be written to size best-quality-possible thumbnails to context-appropriate ones as required (IIRC v11 is coded like that, very large recommended thumbnail size, as per the way the web has been going).
My understanding is 0.jpg is the best you can get.

I'm going to close this for the above reason. But if I'm happy for it to be reopened if I am missing something, which I may be.