Recently one of the guys behind Spitfire Audio, Christian Henson, started his own channel. I must say I find it very interesting to follow his hectic life as a composer and sample library developer. He does some very creative video editing to keep things interesting.
Each video is just a few minutes, but still very inspiring and interesting. His channel is here.
From my expirience I can say that best channels for composers are from developers of plugins and VSTi
Just go to ProjectSam channel or FabFilter,those guys explain in 10-12 minutes more than other channels can tell in 2-3 hour video
Also great channel is Think Space Education https://www.youtube.com/channel/UCMHG0rJtVF1LohiP63FJakw