What is the easiest (fastest) way to get direct link to MP4 video on YouTube?

I don't think it's much faster. I use youtube-dl from MacPorts or Homebrew.

> youtube-dl -g https://www.youtube.com/watch?v=MkmyLTPh3UY
http://r7---sn-nfpnnjvh-1gil.c.youtube.com/videoplayback?itag=34&expire=1357448601&source=youtube&factor=1.25&sver=3&ip=178.193.24.93&algorithm=throttle-factor&ms=au&key=yt1&mv=m&upn=tGspI9WJELc&burst=40&id=3249b22d33e1dd46&mt=1357423931&cp=U0hUTFRPVl9FS0NONF9MSVpHOlp2Yi1jNlUyeUdG&fexp=914501%2C919358%2C910207%2C929206%2C916611%2C920704%2C912806%2C928001%2C922403%2C922405%2C929901%2C913605%2C929104%2C929109%2C929110%2C913546%2C913556%2C908496%2C920201%2C913302%2C919009%2C911116%2C901451%2C902556&ipbits=8&gcr=ch&sparams=algorithm%2Cburst%2Ccp%2Cfactor%2Cgcr%2Cid%2Cip%2Cipbits%2Citag%2Csource%2Cupn%2Cexpire&signature=910FFEBF23CC5F88185DB98028C346DF76EF83C3.2B57D82069876F73C1E8F886AEFFDF2F2039F7F5

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Install the ClickToFlash extension in Safari. With the extension enabled you can right-click on any video in YouTube and choose "Download File", which downloads the underlying MP4 file.

The plugin also has the side benefit of blocking Flash content by default, but if you don't want this functionality this plugin might not be for you.

I personally use Chrome, and there is a great (unofficial) extension, which allows you to set both the default size/resolution to use for downloads, but also gives you a choice for a different resolution for each video you visit. Extremely handy.

I also use the YouTube Options Chrome extension to disable the initial buffering (so videos don't buffer until I click play). I know your original post was looking for a Safari solution, but this combination works great so I thought I would share.