My absolute favorites:
Art of the Problem (Information Theory/Cryptograpghy) : https://www.youtube.com/user/ArtOfTheProblem
Historia Civilis (History) : https://www.youtube.com/channel/UCv_vLHiWVBh_FR9vbeuiY-A
3 Blue 1 Brown (Maths): https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw
Primitive Technology (Survivalism) : https://www.youtube.com/channel/UCAL3JXZSzSm8AlZyD3nQdBA
PBS Space Time (Physics) : https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g