vlogbrothers: a channel shared by two brothers, John and Hank. They take turns making each other videos as a way of avoiding text-based communication. John is an author, Hank is a musician, and both are self-avowed nerds. They've become insanely popular, with an entire community of "nerdfighters" rallying around them. They've spawned two other channels that I also watch; crashcourse, where John teaches history and Hank teaches biology, and scishow, a science news show by Hank.
Lindseythenerdiest: This is another vlogging-style youtube channel. I actually found her through vlogbrothers and have been watching her ever since. Nothing fancy, just a funny, articulate, young woman ranting to the camera.
Elmify: this vlog's amusing twist is the infusion of different personalities; there's Elmify, her interrupting t-shirt-making alter ego, angst-ridden teenage Elmify, and pony-tail wearing child Elmify. All these are played by the same woman, who provides slices of her life with a layer of humour.
TEDtalks: I'm including this here because I used to really like it. I would devour every video when it really was ideas worth sharing. Videos about scientific breakthroughs, advances in medicine and engineering and biology, ideas that could change the world. A 20-minute powerpoint presentation (100 years by youtube standards) seemed to pass in the blink of an eye. Now TED's videos have lowered considerably in quality. Now they have talks about nothing in particular from whoever has been popular on the internet.
Sesamestreet: I may spend the most time watching this channel, although not always by choice. All your favourite (and some not so favourite) Sesame Street skits are online. Since the show is made up of short segments, it really lends itself well to youtube. I'm partial to Cookie Monster and Grover, while my daughter enjoys Elmo (blech) and Abby Cadabby.
Here's one I enjoyed from Collegehumor, Dinosaur Office. I'm not sure why it's so hilarious:
And finally, one by my brother which my son and I have enjoyed for years: