It's hard to name just one channel (especially considering they cover such vastly different topics and styles), but I suppose a fair way of choosing is by most time spent on. In that case, my favorite channel is "The Young Turks" (
https://www.youtube.com/user/TheYoungTurks) - it's a news channel, but it has both very high quality content, and is very entertaining (definitely worth to follow).
Other than that, there are several other channels I like to keep an eye on (in no particular order):
https://www.youtube.com/user/TheAmazingAtheist - The Amazing Atheist, and his ramblings on, well, just about anything.
https://www.youtube.com/user/penguinz0 - very amusing reviews of, less than well known, games by penguinz0.
https://www.youtube.com/user/Vsauce - at times weird (in a good way), but always very interesting content.
https://www.youtube.com/user/RichardDWolff - RichardDWolff and his monthly presentations on Global Capitalism.
https://www.youtube.com/user/thejuicemedia - thejuicemedia with their critic/commentary on current events (some of the videos are hilarious).
https://www.youtube.com/user/TheRealNews - The Real News Network: another news channel, but also very good and informative.
https://www.youtube.com/user/crashcourse - they have a series of mini-courses on a variety of subjects (and new ones coming out frequently).
https://www.youtube.com/user/minutephysics - quick and interesting physics related videos.
https://www.youtube.com/user/sixtysymbols - they periodically cover Astronomy and Physics related subjects (in a simple and easy to understand way).
