History:
Schola Gladiatoria
Lindybeige
Skallagrim
Knyght Errant
Alternative History Hub (speculation about the effects of different outcomes in the past, e.g. Carthage defeating Rome)
Historia Civilis (awful sound quality but good content)
Science (for non-scientists) and other educational stuff:
Sixty Symbols
Objectivity
Veritasium
Scishow and Scishow Space
Fraser Cain
It's OK To Be Smart
Kurzgesagt - In a Nutshell
Tom Scott (linguistics videos and "Things you might not know" videos)
Nativlang
Cody's Lab
Cars:
Motor Trend
Bad Obsession Motorsport (a video every couple of months, but they're excellent)
Might Car Mods
Visioracer (list-style compilations of snippets that can be interesting, such as cars refitted with a very different engine to the one they came with).
Tom's Turbo Garage
Comedy:
Bad Lip Reading. Funniest thing on Youtube in my opinion.
Tom Scott (Citation Needed videos - hilarious comedy quiz show)
Shoddycast (the "Science!" videos analysing how realistic various things in various games are, which is usually not at all. NSFW due to swearing.)
Smosh Games (particularly the "Honest Game Trailers" videos).