One Piece is the most charming, heartwarming, heartbreaking, unique, well-thought out fiction I have ever read. It was life-changing for me to be honest. It manages to be hilarious while teaching amazing lessons, heartbreaking while still instilling feelings of hope, have a unique art style while still being visually stunning, etc.
It's often been said that the characters don't develop at all, and while I understand that claim, I don't agree. Characters like Robin manage to do a 180 throughout the course of the series while still retaining the core traits that made you like them in the first place.

Every moment of One Piece for me is pure joy.
The best way I could describe what makes it stand out from other long-running shonen is that to me, in series like Naruto, you believe Naruto will become Hokage because he is the main character.
In OP, you believe Luffy will become the pirate king because he is Luffy.
OP has the best world building out of any story I've ever read, a unique and well-written cast of characters with goals and morals as diverse as they are, and a story that still manages to be engaging after 20+ years.

As much as I may love this series, I am not blind enough to think everyone will enjoy it. If you are new to the series, I would recommend reading up to the end of the Arlong Park arc (about 8 volumes in). At that point, you can get a feel for the rest of the series will be like, and decide if you want to continue.

I'm still stunned that a manga that I only rediscovered about a year ago has managed to change my life so much. I can definitely say that OP has changed my life for the better.
If you read it, that's cool! If you aren't convinced, that's cool too! Either way, thanks for reading this far!