Pretty much echoing everyone else, in so far as Super GT and Viper Concept
I am mentioning those 2 as I think they are the ones I watch the most.
Others, who aren't totally focused solely on GT Sport, but who are worth a watch if you enjoy sim racing in general, are Jimmy Broadbent, Empty Box, and Tiametmarduk.
Jimmy and Box focus more on the hard core side, with expensive rigs, wheels, and proper I-Racing, R Factor2, Assetto Corsa level, full sim physics, tyre models etc...
Tiametmarduk is primarily known for his f1 game content, but does other motorsport games.
