Handmade hero [1] mostly, and sometimes Jonathan Blow's compiler programming streams [2].
Not all of these video's are equally interesting, but there's some real gems in there, e.g. where Casey (of HH) hand-optimizes his bilinear bitmap interpolation code (somewhere between streams ~115 and ~120 if I remember correctly).
I cringe a lot about their coding style and complete disregard of architecture and basic code hygiene they appear to have, but there's still a lot of interesting things to learn from watching these streams.
Which ones?