The_Chieftain, history concentrating mainly on tanks. His Inside the Hatch series, aka Does He Fit, has revolutionised how tanks are seen, as did his lecture on Myths of American Armor. Operation Think Tank is highly enjoyable as well. "Drive me closer. I want to hit them with my sword."
TIK. Concentrates on the more obscure aspects of WW2, and actively encouraging revisionism as a way of re-examining the historical evidence. His recently posted re-examination of Dan Pienaar during Crusader, in response to a critic's belittling of his knowledge on the subject, is a masterpiece of historiography.
Also look at Military History (not) Visualized, which are two channels, Visualized and not Visualized. Extremely informative if you're already versed in the subject, but extremely dry if you don't already know a fair bit.
Bookmarks