Without saying anything negative about D&D 5e, let me tell you about two of my personal favorites:

The Dark Eye

Under the name "Das Schwarze Auge", this is one of the most popular systems in Germany and has existed since the mid 80s and the latest edition has been available in English for about a decade now. There are dozens of source books and hundreds of official campaigns and standalone adventures, all set in the same world and a single ongoing canon (apart from a few early works that have been retconned). There are decades of detailed in-world history that you can use as a background for your own campaign if you want or selectively ignore if you want to focus on your own interpretation of what the world should look like.

Mechanics-wise it's a lot less board-game-like than some 70s/80s/90s systems while not going the full "storytelling first" route that many more moderns systems seem to prefer. On top of the eight basic attributes, characters can select from a pool of skills and feats that cover everything from combat to magic to social interaction to crafts and hobbies. The system focuses a lot less on combat than other high fantasy systems and it's absolutely viable to have a group of purely social-focused characters that never get into a single fight but still get to use a lot of the system's mechanics.

Overall it's relatively complex if you want to use absolutely every rule but at the same time very versatile and can be customized to your playstyle.

Opus Anima / Opus Anima Investigation

Sadly out of print and never officially translated to English so I'll focus on the one thing that works without the official setting: it's one of the simplest systems I've ever seen. It uses a pool of D2s (odd/even on D6, coins, red/black cards, whatever you have on hand) where the number of dice is determined by a basic attribute and a skill that can be combined however the situation requires. Dexterity + mechanics to build something, perception + mechanics to recognize a mechanism, knowledge + mechanics to understand the underlying principles or remember who invented something. To avoid experienced characters failing an easy check out of pure bad luck, everything over 10 dice is not rolled but gives half a success (rounded up) automatically. That's it. That's the whole system.