It depends on what you're looking for! I'll give just a handful here, but I'm happy to expand if you have a specific follow-up questions to these.

If you're interesting in something very D&D-like, there's obviously Pathfinder 2E[1], which builds on the same D&D skeleton but has a much sharper and cleaner approach to grid-based tactical combat. However, I'd also suggest looking at some of the games in the OSR ("Old School Renaissance") space. Games like The Black Hack[2] or Maze Rats[3] provide a much smaller set of rules which are easy to adapt to other adventures: the goal is that you can take adventure modules for effectively any existing D&D-like game—including both present and past versions of D&D—and run them with little overhead.

Something pretty different mechanically but which is quite compelling in that space is Torchbearer[4], which is based on the underlying Burning Wheel[5] system but made significantly simpler (and shares a lot of those simplifications with Mouse Guard[6] except it's, well, not about sword-wielding mice.) Torchbearer is a great dungeon-crawl-focused system that can really capture grit and difficulty in a way that's a lot of fun, but it's also the kind of game where you can get a total party kill not just by a dragon but also by running out of food and torches, so expect a grimy tough game out of it!

If you're looking for something even further afield, I'd suggest taking a peek at Dungeon World[7], which borrows the core mechanics from indie darling Apocalypse World[8] but applies them to a traditional D&D milieu: that said, I'd actually start with Homebrew World[9], which streamlines and clarifies a lot of the rules, but it might require Dungeon World itself to get a handle on how to run the game. Games inspired by Apocalypse World—sometimes called Powered by the Apocalypse games—definitely play a bit differently—they tend to be a bit more "zoomed-out", e.g. combat being resolved in a fewer high-level rolls rather than playing out a full sequence of six-second slices like D&D—and they aren't to everyone's liking, but I think they're worth trying.

I'm also going to plug my personal favorite tabletop game, Blades in the Dark[10], which is not a traditional fantasy game (although people have adapted the the rules to more traditional fantasy, c.f. Raiders in the Dark[11]) but which I think is super compelling. It's about criminals in a haunted Victorian-ish setting doing odd jobs, and builds a system that's top-of-its-class for doing that, including mechanical support for heist-movie-style flashbacks and a lot of systems designed to let you do risky moves and narrowly avoid failure from them. Some of my absolute favorite TTRPG moments have been in Blades games.

Any of that sound interesting? Want other examples or directions?

[1]: https://paizo.com/pathfinder
[2]: https://www.drivethrurpg.com/product/255088/The-Black-Hack-S... with the open content collected at https://the-black-hack.jehaisleprintemps.net/
[3]: https://www.drivethrurpg.com/product/197158/Maze-Rats
[4]: https://www.burningwheel.com/torchbearer-2e-core-set/
[5]: https://www.burningwheel.com/burning-wheel/
[6]: https://www.mouseguard.net/book/role-playing-game/
[7]: https://dungeon-world.com/
[8]: http://apocalypse-world.com/
[9]: https://spoutinglore.blogspot.com/2019/05/homebrew-world-v15...
[10]: https://bladesinthedark.com/greetings-scoundrel
[11]: https://smallcoolgames.itch.io/raiders-in-the-dark