Currently? GURPS. It's such a fascinating TTRPG. On one hand, it's extremely simple (roll 3d6 ≤ skill level + modifiers). On the other hand, it has an encyclopedic catalogue of skills and modifiers to accommodate anything from cavemen to reality-warping future civilizations. If you want, it can be extremely gritty and detailed, more than whatever you're thinking of right now; or dead simple and narrative driven

The ridiculous breadth of options make it look really complicated, so people get intimidated. I think I'm it's hard for people to wrap their head around just how optional all the granular rules are.

programming problems, retro tech stuff, music in depth (e.g. theory level), star trek, half-life and mother series

Late 90s early 2000s Japanese mixed martial arts and kickboxing. Pride, pancrase, k1. People still talk about the UFC today, i just think "you don't even know..."

Major League Baseball history. Old time teams, players and stats, the broader historical context with its' events, attitudes and quirks... the whole thing.

Digital mixers. I know a lot of people who use them and know them but very few know how they work internally which is fascinating and interesting and the only people who do are under NDA's