ttrpg nerd bait, my favourites:
Forbidden Lands: This is a whole old-school D&D exploration-based game. It's a big, huge setting with a lot of content to explore. Characters are simple to make, the rules are light, and it's easy to run with many groups or players dropping in-and-out without ruining the continuity of any stories.
Microlite20/Microlite70: These are super-small, cut-down, and free revisions of the old-school D&D style games. The rules fit in a couple of pages. It's all compatible with the enormous library of OSR content out there. And it's straight-forward to hack into your own setting, add your own house rules, etc.
Torchbearer: If you like the idea of down-on-their-luck mercenaries out on the edges of civilization robbing ancient tombs and hunting down relics for rich patrons in order to make a buck and scrape by; this game is great. It has the classic D&D feel of regular people picking up a sword and heading into the wilderness with plenty of story-telling hooks. Gameplay is much more structured than in Microlite* games but it's also very easy to prep for as a DM.
Burning Wheel This is the grand-pappy of Torchbearer (and other games made by the same publisher like Mouse Guard). It's a crunchier system than most as it has rule systems for debates, all kinds of combat, etc. However this game is more of a world-building toolkit kind of RPG where you are encouraged to build your game world and story at the table and pick which parts of the rules will be relevant to your game. Want a game with high political intrigue in a low-tech society with no magic but faith? You can use the rules for debating (called, Duel of Wits), skip on all the magic and creatures and limit characters to humans and faith-based paths. Want a high-magic setting with monsters lurking around every corner? You can build that too.
It's a bonus that the books are handsome and well made.
Root/Ironsworn/Powered by the Apocalypse: These are all games that use the same basic mechanics. More narrative-driven and light on the rules/mechanics. Generally pretty cheap to acquire and teach.
Wraith: The Oblivion 20th: My favourite World of Darkness setting. Very much not D&D... but still a TTRPG. I tend to play Vampire more because it's more popular. However this one is an overlooked gem of a game and setting. It's all about playing as ghosts and the afterlife. However what sets it apart from the rest of the World of Darkness setting is that it's possible to transcend from the world and have a good ending. This system, I find, is pretty rules-light and easy to get into... although it's the content of the game that's overwhelming! There are a lot of options for building characters and it will take a long time before you get a grips on all the different options. However stories here tend to be dark, dramatic, sometimes hilarious... it's worth making sure you're playing with the right group that can handle the subject matter and support one another playing through it.