Does anyone have any idea on the ranking algorithm of the big tube sites?
 

Is it similar to YouTube, where it's mostly about title, description, tags, and user engagement metrics? Pretty interested in optimizing my videos for some keywords :)

I would imagine a large part would be time on site plus traffic plus bounce.
Title and description are always important.

What ever the search engines pick up. The more information the better. Sure some tubes have a tonne of own traffic but is the SEO content that will be the reasoning for ranking order like you describe. Unless random or last post etc.