For YouTube, the Leaders can be split into categories.
History: Jago Hazzard - needs little introduction but everyone enjoys the wit, history, and they are nice videos to sit down and to watch whatever mood you are in tbh.
Modern rail - i mean that has to go to Geoff Marshall

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Steam and engineering: Chris Eden Green is good quality content as it does tackle some of the realities behind steam preservation but also makes learning about the technical rather than the face value of locos more interesting aswell. My favourite has to be Lawrie from Lawries Mechanical Marvels as its easy watching and he does a very relaxed way of showing you how a steam/diesel loco is prepared for service on and his enthusiasm for his collection or the loco he is working on is brilliant

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I would simply be parroting if i went on about the same Youtubers so here is a couple i think deserves some attention.
In the history section, i think Doncaster Drawn is a good emerging channel as if you liked Jago stuff its a recommended watch just a lot less London stuff. The best video she did in my opinion was about A2 Blue Peter which is rather overlooked these days owing to New build A1 Tornado.
For preservation i will definitely recommend the Channel for the Talyllyn railway as they have done small mini documentaries on their locos and they have been keen to cover the coal crisis on their channel and trialing new fuels on there.
Honestly surprised Ruairidh Macveigh hasn't come up on this thread yet, then again he is doing alright for himself. Same for the rediscovering lost railways channel too.