The answer is probably 'the last one I watched'.
Someone linked to his scraping video for the 'making an accurate vice' thread but I think these are all so good they deserve a special mention in Online Resources.
https://www.youtube.com/user/dgelbart/videos
I wish I was Dan. I want a waterjet – and all the other tools he has at his disposal. So far, I have found something new in every video. I think the one that opened my mind the most, so far, is Flexures.