I’d like to restrict this to “amateur” videos with relatively few viewers. There’s plenty of highly financed stuff on there, and even some amateur stuff that gets a lot attention (like the truly excellent Khan Academy) but I think there are plenty of knowledgeable people with just a camera and mike who deserve some more attention.
My current favorite of these is LithodidMan’s 1 and half year (for now) spanning series on Sexual Biology. He’s a professional biologist who’s obviously good at explanations, sometimes very opinionated, and a hugely entertaining speaker. I’d recommend this particular series to anyone remotely interested in the evolution of sex, and most of his other videos for people who are interested in evolution in general.
So, post your recommendation - preferably with some introduction/explanation why it’s so good.