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 David 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.