Because I manually download videos to watch on the bus or train (thanks Grayjay & NewPipe), most of my subscriptions are for long-form content:

Retro Tech: Techmoan, Technology Connections, Posy, Janus Cycle, CRD, Ben Eater, DiodeGoneWild, pannenkoek2012, videolabguy, Adrian’s Digital Basement,The Science Elf, previously LGR and 8-Bit Guy/Keys

Science: Kuvina Saydaki, BobbyBroccoli, Numberphile, Computerphile, carykh

Tech News & Discussion (not always long-form): Louis Rossmann, Mental Outlaw, Brodie Robertson, SomeOrdinaryGamers, Asianometry, Atomic Shrimp, previously Thunderf00t

Urbanism: Not Just Bikes, Adam Something, Alan Fisher, Tramly, BritMonkey

D&D Story Narration: CritCrab, Puffin Forest

Bold channels are most underrated imo

If you don’t want to use the potentially unstable Nightly, Dev or Beta, you can use Fennec (stable builds with dev features).