One piece easily. I'm not really a huge fan of FT but I don't think its the worst manga of all time, especially when shit like Eromanga sensei exists, it definitely has some pretty cool character designs though.
The world building in One Piece is really interesting. There’s a lot going on. Plot teases, characters, and references return hundreds of chapters later with full blown story arches. If you’re patient, it’s really satisfying.