Some that hadn't been mentioned yet (at least when I started writing this) :)

CuriousMarc has some stunning repair/teardowns of old kit, including a long series on the Apollo Guidance Computer: https://www.youtube.com/channel/UC3bosUr3WlKYm4sBaLs-Adw

Juan Browne's Blancolirio channel has interesting insights into most air crashes and near misses: https://www.youtube.com/user/blancolirio

BobbyBroccoli has created some stunningly high quality videos on scientific scandals and controversies: https://www.youtube.com/user/BobbyBroccoli

Captain Disillusion has fantastic insights into CGI and video manipulation, presented in a rather unique style: https://www.youtube.com/user/CaptainDisillusion

CGPGrey produces high quality, informative and quirky videos about all sorts of things (why runways have the numbers they do, the history of the name Tiffany). Always well written with a pleasing use of language: https://www.youtube.com/user/CGPGrey

CodingSecrets has been commercially coding on consoles for 30 years and has some great insights into how old games work - sega especially https://www.youtube.com/channel/UCkY047vYjF92-8HcoVTXAOg

ConnectionsMuseum is a must watch if you're into early telephone exchanges (or want to understand where half the terminology and timers come from in SIP). They have a handful of different vintange telephone switches and go quite in depth explaining how they work: https://www.youtube.com/user/museumofcomm

Frame Voyager has a fantastic series on "abandoned" cinema cameras. Well presented, and interesting if you're into film and tech: https://www.youtube.com/channel/UCmXGDFnFh95WlZjhwmA5aeQ

Hbomberguy is... something else, especially in presentation style (personally I love it). His "Vaccines and Autism, a Measured Response" video is stunningly informative and presents more information into the Andrew Wakefield scandal than any other documentary on the topic. https://www.youtube.com/user/hbomberguy

HowNOT2 started out as slacklining, but have spent a lot of time deliberately breaking and abusing rock climbing gear in various ways. https://www.youtube.com/channel/UCQvq-0fss4lNrmIz7gcPLtQ

Ian Hubert has created a couple of short but absolutely unbelievably amazing videos in Blender. A must watch if you're interested in 3D, green-screen or scifi. https://www.youtube.com/user/mrdodobird

Isaac Arthur: Science, space, futurism. https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g

John Ward does some dry but extremely informative mains electrical theory and regulations (UK centric): https://www.youtube.com/user/jjward

Jay Foreman: Silly educational videos: https://www.youtube.com/user/jayforeman51 (also find "Map Men").

Karl Jobst: Video game speedrunning and controversy: https://www.youtube.com/user/karljobst

LindyBeige: History of war and weapons: https://www.youtube.com/user/lindybeige

Photonic Induction: Think ElectroBoom, except this nutter blows up 400A fuses. High voltage silliness: https://www.youtube.com/user/Photonvids

Ringway Manchester: (Ham) radio related: https://www.youtube.com/user/RINGWAYMANCHESTER

Sabine Hosselfelder: Science updates without the hype or spin: https://www.youtube.com/user/peppermint78

This Old Tony: Stunningly high quality, dry humour machining: https://www.youtube.com/user/featony