MrMooseBones (3.5k subs)
https://youtube.com/user/MrMooseBones
Competitive. Monster. Rancher. 2. Sounds strange, and I'm not sure how the algorithm recommended this one to me, but it's fantastic content. Tournaments, strategy, a Discord, metagame analysis, the whole shebang. Monster Rancher is having a moment right now due to the 1+2 DX re-release and Nash is at the forefront of it.
Stefan Ash FFXIV (8.1k subs)
https://www.youtube.com/c/StefanAshFFXIV
Helpful, practical advice for FFXIV. Covers a wide range of topics like uncommonly used UI settings, guides to playing each class/role, and general advice surrounding the game.
Anton Hand (90.5k subs)
https://www.youtube.com/c/AntonHand
The guy behind Hot Dogs, Horse Shoes, and Hand Grenades. Mostly dev logs, lots of great insight about real firearms, VR, and gamedev. Before anyone asks re: gun stuff, Anton is on the far left politically.
Jeremy Parish | Video Works (63.2k subs)
https://www.youtube.com/c/JeremyParish
Probably not very obscure to people around here, but still under 100k. Jeremy Parish has spent years exhaustively documenting the history and libraries of various game systems in his fantastic Video Works series. Also has some series about topics that interest him personally, like gin and the history of Metroidvanias.
Highlight Reel (43.4k subs)
https://www.youtube.com/c/HighlightReel
Standalone continuation of the Kotaku series. A weekly compilation of user submitted videos that are usually funny or impressive.
Classics of Game (16.8k subs, currently inactive)
https://www.youtube.com/user/ClassicsOfGame
A YouTube haiku collection. Short game clips that are often funny or absurd, featuring lots of PSX budget releases.
Illusory Wall (93.7k subs)
https://www.youtube.com/c/illusorywall
Souls series analysis, secrets, mythbusting, out of bounds exploration, etc.
Honorable mentions over 100k:
Retro Game Corps: One of the "holy trinity" of retro handheld channels.
ThorHighHeels: Documentary style videos mostly about weirder, more obscure Japanese games and the ephemera surrounding them.
Food4Dogs: A very nice older lady who got into JRPGs, particularly the Atelier series.
strafefox/Splash Wave: Deep dive gaming documentaries with a technical emphasis.