For fantasy RPGS, my games are ran with a chimeric set of rules.
At its core, Ben Milton's Knave forms the engine (min(3d6) for ability modifiers, CON+10 item slots). The magic system was pilfered from Dungeon Crawl Classics (DCC). Zero spell slots, spell burn, and cast as often as you like with risk of misfire, but tiered successes. As for everything else, a smattering of Gavin Norman's Old-School Essentials (OSE), OSR blogs/zines, The Angry GM, and Fiasco for creating fun and silly PC relationships. And of course, a heavy dose of improv as well.
This is my second iteration as a DM. Between 2017-2019, as a first-time DM, I ran twenty-one sessions of D&D 5e for three different parties: Legendary Heroes of Fun & Awesomeness, Blood Hawks, and Brow & Co. I was quickly overburdened with rules bloat. Prepping modules felt like a chore. And 5e's lengthy stat blocks overwhelmed me. Today, my stat blocks look like this:
Goblin: AC: 13, HD:1(hp:4), ATK:1(1d6) ML:8
Everything else is made up based on context. In short, I hate accounting and I am an OSR convert. The simpler the rules, the more likely players stop looking at their character sheets for answers and use their imagination instead. As a DM, this also allows my NPC and monster reactions to be more organic and "within the narrative".
Dice rolls are kept to a minimum and rolled in the open, which also means there is no "game balance". Players must learn to assess risk and reward. Rash and unwise decisions (such as rushing into combat without tactics) should be punished, as in real life.
I keep a record of dead player characters in The Graveyard. My players reading this should visit to pay their respects.