For pure teach Nithrania - Game in a Nutshell and JestaThaRogue are my favorites. For playthrough based teaches JonGetsGames is good.

Before You Play is kind of a hybrid, they often do a teach followed by a playthrough. BGG's gamenight (have to search for "gamenight") often has good teaches and playthroughs, especially when it's Dave Arnott's turn to teach. I also enjoy Slickerdrips' playthroughs, the teach in these is a bit more loose but still helpful.

When you are in a hurry 3 Minute Board Games or The Rules Girl can be helpful but these are not as thorough.

JonGetsGames, hands down!

Gaming Rules is absolutely fantastic.

Board Game Tutorials is probably the best one you have never heard of.

High production values, excellent teach and he puts on funny voices.

Only 22 videos so far, but it's a high recommend from me!

Good suggestions in replies. Watch it played is actually my last option. Jongetsgames, Before you Play, Nithrania are all good. Shout out to Taylor's Trick Taking Table for TT fans.
I dread when I have to see Heavy Cardboard because no one else has a video for the game.