Great thread! I've thought about posting one like it because I'm always looking for new channels with great content. Here are some of my favourites:
3Blue1Brown: My favourite math channel. I've learned a lot from it.
All Things Comedy: There are a few funny shows but Bert Kreischer cooking with other comedians is always gold.
Amoeba: Amoeba Records gets musicians, and other famous people, to show us what's in their bag. It's a great way to see what influences them.
Carfection: This is what Top Gear should have become after Clarkson, Hammond, and May left.
KEXP: KEXP is an amazing radio station that constantly gets great bands to come in and play sessions.
quill18: quill18 does Let's Plays of strategy games, and he's a cool Canadian.
Virtual Railfan: There's something zen about trains, and this channel will bring out your inner "foamer". Even if you don't like trains you'll like seeing idiots trying to deal with them.