Matt Brown - security researcher and bug bounty hunter

Robert Murray Smith - general mechanical engineering tinkering, lots to learn

TheCod3r - console and other electronic repairs

Wesley Kagan - builds cars but is just an overall genius and entertaining

Joe Grand - hacker, genius

There are more popular/sometimes better people and channels in each genre but I've stuck to those with under 1 million sub's. I'll assume people like Colin Furze, Ben Eater, Computerphile are already too well known for this thread