Here's why:

1. Rich lore, the stories, the historical references used are crazy amazing, you'd be surprised w Oda's knowledge when you understand it all, it's on level of Isayama if not, even better than him, rather wayyy better than him, no joke

2. It's extremely entertaining, the humour, the characters and their personalities along w character development is on point along w dialogues, you'd find yourself laughing a lot of times subtly and love all the details, no two characters will look the same like the creativity of the author is on peak, you'd be constantly looking forward to things ki what will I get to see next, it's the best seriously, and there's so much effort put into it, true definition of quality + quantity! All side characters get their spotlight and their battles aren't half based, the creativity even in battles is amazing, from their back stories to songs made by him which pirates sing, like it's all just beautiful, I constantly zoom in when reading cuz I love the details oda puts in

3. World is ever expansive, it doesn't leave any possibilities any theories, everything counts, it'll keep on expanding and satisfy your imagination, it's extremely creative w the world, best world building, ever, ninjas, samurais, pirates, dinosaurs, angels, mermaids, magical creatures, cyborgs, ghosts, fairies, zombies, fishmen, normal ppl, haki/devil fruit users, giants, you call it, it'll all be there in one piece

4. Best side characters to ever exist in an animé, everyone gets their moment to shine, this is where Naruto failed and had to constantly add fillers to get them covered up, it feels forced, DB is all about Goku and Vegeta now, except for One Piece, each and every side character, from the most hated villain to your favourite character, everyone does come back at one point or the other which makes you feel they genuinely exist in the world, they're not some side character who'll appear and be cut off forever and in the manga on the first page as the current events are going, what's going on w side characters is also shown and it's really entertaining and good to see, at the end of the ch there's a SBS corner where Oda answers questions of the fans and its really funny/informative, love it

5. A lot of touching moments, even thinking about them gets me goosebumps, like every arc makes me emo and want to cry lol, there are few songs and stuff which I YouTube it cuz can't listen to them in manga obviously, but yeah it's love, one piece also has the most original laughs ever, like many characters will have different laughs and I tune into anime just to listen their laughs lol everything hits hard in one piece, let it be Luffy's punches or someone's death, the sadness, the grief, the portrayal of pain, it all feels too relatable, you'd find yourself crying at some boat's funeral, or during an epic fight or their speech, it'll make you scream, it'll make you feel, it'll get tears in your eyes and make you yell "GO LUFFY!!!!!" Man, just thinking about it got tears in my eyes and goosebumps on my entire body, peak creativity, the author knows how to give us a satisfying end, the build up and the last attack which finishes a battle are intense and really satisfying

6. The mysteries, one piece has a lot of mysteries, they actually feel like the world's secret, you'll be curious and keep on wondering what's that, what will happen if this happens, poneglyphs, 800 years void century, imu, legendary 5 elders, the will of D, ancient weapons, "one piece", true history and so much more, and a lot manyyyyy of them are unanswered, so you see there's going to be a lot of stuff incoming, a lot of stuff still to be revealed like there's so much to look forward to and there's so much hype and ppl love it still so much even after 1000 chapters, it never lost the pace or disappointed the fans. One piece is a true example of quality + quantity, endless fun, and I hope it never ends, cuz it's just amazing, one piece is godly and nothing comes close to it.