What can you say about One Piece that hasn't been said before?
It is unbelievably imaginative, with a massive amount of characters, settings, and concepts, that have almost never been seen from a single work, and which could technically supply enough material for another 20 years or more.
The main cast was always likeable, but it is first with the Shabaody Archipelago arc that the series really started to shine, due to that from there onwards it starts to change the pace by focusing on the large, also lots of fun, tertiary characters.
The main strength of this series is that you always find something new in it, and never know what to expect, and the art is great, unique, and always entertains you. It makes you enjoy life more, and is one of extremely few works to be considered mainstream for the average Japanese citizen.